Semiconductor device and method for driving semiconductor device

ABSTRACT

It is an object to provide a semiconductor device with a novel structure in which stored data can be held even when power is not supplied, and does not have a limitation on the number of writing operations. A semiconductor device includes a plurality of memory cells each including a transistor including a first semiconductor material, a transistor including a second semiconductor material that is different from the first semiconductor material, and a capacitor, and a potential switching circuit having a function of supplying a power supply potential to a source line in a writing period. Thus, power consumption of the semiconductor device can be sufficiently suppressed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The disclosed invention relates to a semiconductor device including a semiconductor element and a method for driving the semiconductor device.

2. Description of the Related Art

Storage devices including semiconductor elements are broadly classified into two categories: a volatile device that loses stored data when power is not supplied, and a non-volatile device that holds stored data even when power is not supplied.

A typical example of a volatile storage device is a DRAM (dynamic random access memory). A DRAM stores data in such a manner that a transistor included in a storage element is selected and electric charge is stored in a capacitor.

When data is read from a DRAM, electric charge in a capacitor is lost owing to the above-described principle; thus, another writing operation is necessary whenever data is read out. Moreover, since leakage current (off-state current) flows between a source and a drain of a transistor included in a storage element when the transistor is in an off state for example, electric charge flows into or out even if the transistor is not selected, which makes a data holding period short. For that reason, another writing operation (refresh operation) is necessary at predetermined intervals, and it is difficult to sufficiently reduce power consumption. Furthermore, since stored data is lost when power is not supplied, an additional storage device using a magnetic material or an optical material is needed in order to hold the data for a long time.

Another example of a volatile storage device is an SRAM (static random access memory). An SRAM holds stored data by using a circuit such as a flip-flop and thus does not need a refresh operation. This means that an SRAM has an advantage over a DRAM. However, cost per storage capacity is increased because a circuit such as a flip-flop is used. Moreover, as in a DRAM, stored data in an SRAM is lost when power is not supplied.

A typical example of a non-volatile storage device is a flash memory. A flash memory includes a floating gate between a gate electrode and a channel formation region in a transistor and stores data by holding electric charge in the floating gate. Therefore, a flash memory has advantages in that a data holding period is extremely long (almost permanent) and a refresh operation which is necessary in a volatile storage device is not needed (e.g., see Patent Document 1).

However, a gate insulating layer included in a storage element deteriorates by tunneling current generated in writing, so that the storage element stops its function after a predetermined number of writing operations. In order to alleviate an influence due to this problem, a method in which the number of writing operations is equalized among storage elements can be employed, for example, but a complex peripheral circuit is needed to realize this method. Moreover, employing such a method does not solve the fundamental problem of lifetime. In other words, a flash memory is not suitable for applications in which data is frequently rewritten.

In addition, a high voltage is necessary in order to inject electric charge into a floating gate or remove the electric charge, and a circuit therefor is also required. Further, it takes a relatively long time to inject or remove the electric charge, and it is not easy to perform writing and erasing at higher speed.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.     S57-105889

SUMMARY OF THE INVENTION

In view of the foregoing problem, it is an object of an embodiment of the disclosed invention to provide a semiconductor device with a novel structure in which stored data can be held even when power is not supplied, and does not have a limitation on the number of writing operations.

In the disclosed invention, a semiconductor device is formed using a material which allows a sufficient reduction in off-state current of a transistor; for example, an oxide semiconductor material, which is a wide-gap semiconductor, is used. When a semiconductor material which allows a sufficient reduction in off-state current of a transistor is used, the semiconductor device can hold data for a long time.

An embodiment of the disclosed invention is a semiconductor device including: a write word line, a write-read word line, a bit line, a source line, a potential switching circuit, and a memory cell array including a plurality of memory cells. One of the plurality of memory cells includes a first transistor, a second transistor, and a capacitor. The first transistor includes a first gate electrode, a first source electrode, a first drain electrode, and a first channel formation region. The second transistor includes a second gate electrode, a second source electrode, a second drain electrode, and a second channel formation region. The first channel formation region and the second channel formation region include semiconductor materials that are different from each other. The first gate electrode, the second drain electrode, and one electrode of the capacitor are electrically connected to each other to form a node where electric charge is held. The bit line, the first drain electrode, and the second source electrode are electrically connected to each other. One terminal of the potential switching circuit, the source line, and the first source electrode are electrically connected to each other. The write-read word line and the other electrode of the capacitor are electrically connected to each other. The write word line and the second gate electrode are electrically connected to each other. The potential switching circuit has a function of selectively supplying a power supply potential to the source line in a writing period.

Another embodiment of the disclosed invention is a semiconductor device including: a write word line, a write-read word line, a bit line, a source line, a potential switching circuit, and a memory cell array including a plurality of memory cells. One of the plurality of memory cells includes a first transistor which is an n-channel transistor, a second transistor, and a capacitor. The first transistor includes a first gate electrode, a first source electrode, a first drain electrode, and a first channel formation region. The second transistor includes a second gate electrode, a second source electrode, a second drain electrode, and a second channel formation region. The first channel formation region and the second channel formation region include semiconductor materials that are different from each other. The first gate electrode, the second drain electrode, and one electrode of the capacitor are electrically connected to each other to form a node where electric charge is held. The bit line, the first drain electrode, and the second source electrode are electrically connected to each other. One terminal of the potential switching circuit, the source line, and the first source electrode are electrically connected to each other. The write-read word line and the other electrode of the capacitor are electrically connected to each other. The write word line and the second gate electrode are electrically connected to each other. The source line is electrically connected to the plurality of memory cells in a plurality of columns. The potential switching circuit has a function of selectively supplying a power supply potential to the source line in a writing period.

In any of the above semiconductor devices, the second channel formation region of the second transistor preferably includes an oxide semiconductor.

Another embodiment of the disclosed invention is a method for driving a semiconductor device in which a gate electrode of a first transistor, a drain electrode of a second transistor, and one electrode of a capacitor are electrically connected to each other to form a node where electric charge is held, including the step of: supplying a power supply potential to a source electrode of the first transistor, when a potential at which the first transistor is turned on is supplied to the node.

In the above method for driving a semiconductor device, a channel formation region of the first transistor includes a semiconductor material different from a semiconductor material of a channel formation region of the second transistor.

In the above method for driving a semiconductor device, the channel formation region of the second transistor preferably includes an oxide semiconductor.

In the above method for driving a semiconductor device, an operation in which the power supply potential is supplied to the source electrode of the first transistor is performed by controlling a potential of the source line connected to the source electrode of the first transistor with the use of a potential switching circuit.

In this specification and the like, the term such as “electrode” or “wiring” does not limit a function of such a component. For example, an “electrode” is sometimes used as part of a “wiring”, and vice versa. Furthermore, the term “electrode” or “wiring” can include the case where a plurality of “electrodes” or “wirings” are formed in an integrated manner.

Functions of “source” and “drain” are sometimes replaced with each other when a transistor of opposite polarity is used or when the direction of current flowing is changed in a circuit operation, for example. Therefore, the terms “source” and “drain” can also be used to denote the drain and the source, respectively, in this specification.

Note that in this specification and the like, the term “electrically connected” includes the case where components are connected to each other through an object having any electric function. There is no particular limitation on the object having any electric function as long as electric signals can be transmitted and received between the components that are connected through the object. Examples of the object having any electric function include a switching element such as a transistor, a resistor, an inductor, a capacitor, and elements with a variety of functions as well as an electrode and a wiring.

In addition, the ordinal numbers such as “first” and “second” in this specification and the like are used for convenience and do not denote the order of steps and the stacking order of layers. Further, the ordinal numbers in this specification and the like do not denote any particular names to define the invention. Furthermore, the ordinal numbers are used in order to avoid confusion among components, and the terms do not limit the components numerically.

Since the off-state current of a transistor including an oxide semiconductor is extremely small, stored data can be held for an extremely long time by using the transistor. In other words, power consumption can be adequately reduced because a refresh operation becomes unnecessary or the frequency of refresh operations can be extremely low. Moreover, stored data can be held for a long time even when power is not supplied (note that the potential is preferably fixed).

Further, a semiconductor device according to an embodiment of the disclosed invention does not need a high voltage for writing of data and there is no problem of deterioration of elements. For example, unlike a conventional non-volatile memory, it is not necessary to inject and extract electrons into and from a floating gate, and thus, a problem such as deterioration of a gate insulating layer does not occur at all. In other words, the semiconductor device according to an embodiment of the disclosed invention does not have a limitation on the number of times of writing operations which is a problem in a conventional non-volatile memory, and reliability thereof is drastically improved. Furthermore, data is written depending on an on state and an off state of a transistor, whereby a high-speed operation can be easily realized. In addition, there is no need of an operation for erasing data.

Since a transistor including a material other than an oxide semiconductor can operate at sufficiently high speed, a semiconductor device can perform an operation (e.g., reading data) at sufficiently high speed by combining such a transistor with a transistor including an oxide semiconductor. Further, with the use of a transistor including a material other than an oxide semiconductor, a variety of circuits (such as a logic circuit or a driver circuit) which are required to operate at high speed can be realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1, 1A-2, and 1B are circuit diagrams of semiconductor devices.

FIG. 2 is a circuit diagram of a semiconductor device.

FIG. 3 is a circuit diagram of a semiconductor device.

FIG. 4 is a timing chart.

FIGS. 5A and 5B are a cross-sectional view and a plan view of a semiconductor device.

FIGS. 6A to 6G are cross-sectional views relating to manufacturing steps of a semiconductor device.

FIGS. 7A to 7E are cross-sectional views relating to manufacturing steps of a semiconductor device.

FIGS. 8A to 8D are cross-sectional views relating to manufacturing steps of a semiconductor device.

FIGS. 9A to 9D are cross-sectional views relating to manufacturing steps of a semiconductor device.

FIGS. 10A to 10C are cross-sectional views relating to manufacturing steps of a semiconductor device.

FIGS. 11A to 11F are views each illustrating an electronic device including a semiconductor device.

FIGS. 12A to 12E are views each illustrating a crystal structure of an oxide material.

FIGS. 13A to 13C are views illustrating a crystal structure of an oxide material.

FIGS. 14A to 14C are views illustrating a crystal structure of an oxide material.

FIG. 15 is a graph showing gate voltage dependence of mobility, which is obtained by calculation.

FIGS. 16A to 16C are graphs showing gate voltage dependence of drain current and mobility, which is obtained by calculation.

FIGS. 17A to 17C are graphs showing gate voltage dependence of drain current and mobility, which is obtained by calculation.

FIGS. 18A to 18C are graphs showing gate voltage dependence of drain current and mobility, which is obtained by calculation.

FIGS. 19A and 19B are views each illustrating a cross-sectional structure of a transistor used in calculation.

FIGS. 20A to 20C are graphs each showing characteristics of a transistor including an oxide semiconductor film.

FIGS. 21A and 21B are graphs each showing V_(g)−I_(d) characteristics after a BT test of a transistor that is Sample 1.

FIGS. 22A and 22B are graphs each showing V_(g)−I_(d) characteristics after a BT test of a transistor that is Sample 2.

FIG. 23 is a graph showing V_(g) dependence of I_(d) and field effect mobility.

FIGS. 24A and 24B are a graph showing the relation between substrate temperature and threshold voltage and a graph showing the relation between substrate temperature and field effect mobility, respectively.

FIG. 25 is a graph showing XRD spectra of Sample A and Sample B.

FIG. 26 is a graph showing the relation between off-state current of a transistor and substrate temperature at measurement.

FIGS. 27A and 27B are a top view and a cross-sectional view of a coplanar transistor having a top-gate top-contact structure and including an In—Sn—Zn—O film as an oxide semiconductor film.

FIGS. 28A and 28B are a top view and a cross-sectional view illustrating a structure of a transistor manufactured in Example 2.

DETAILED DESCRIPTION OF THE INVENTION

Examples of embodiments of the present invention will be described with reference to the drawings. Note that the present invention is not limited to the following description and it will be readily appreciated by those skilled in the art that modes and details thereof can be modified in various ways without departing from the spirit and the scope of the present invention. Therefore, the present invention should not be construed as being limited to the description in the following embodiments.

Note that the position, the size, the range, or the like of each structure illustrated in drawings and the like is not accurately represented in some cases for easy understanding. Therefore, the disclosed invention is not necessarily limited to the position, the size, the range, or the like as disclosed in the drawings and the like.

Embodiment 1

In this embodiment, a basic circuit structure and operation of a semiconductor device according to an embodiment of the disclosed invention will be described with reference to FIGS. 1A-1, 1A-2, and 1B and FIG. 2. Note that in each of circuit diagrams, in some cases, “OS” is written beside a transistor in order to indicate that the transistor includes an oxide semiconductor.

<Basic Circuit 1>

First, the most basic circuit structure and its operation will be described with reference to FIGS. 1A-1, 1A-2, and 1B. In a semiconductor device illustrated in FIG. 1A-1, a first wiring (a first line) and a source electrode (or a drain electrode) of a transistor 160 are electrically connected to each other, and a second wiring (a second line) and the drain electrode (or the source electrode) of the transistor 160 are electrically connected to each other. In addition, a third wiring (a third line) and a source electrode (or a drain electrode) of a transistor 162 are electrically connected to each other, and a fourth wiring (a fourth line) and a gate electrode of the transistor 162 are electrically connected to each other. In addition, a gate electrode of the transistor 160 and the drain electrode (or the source electrode) of the transistor 162 are electrically connected to one electrode of a capacitor 164, and a fifth wiring (a fifth line) and the other electrode of the capacitor 164 are electrically connected to each other.

Here, a transistor including an oxide semiconductor is used as the transistor 162, for example. A transistor including an oxide semiconductor has a characteristic of a significantly small off-state current. For that reason, a potential of the gate electrode of the transistor 160 can be held for an extremely long time by turning off the transistor 162. Provision of the capacitor 164 facilitates holding of electric charge supplied to the gate electrode of the transistor 160 and reading of held data.

As the transistor 160, an n-channel transistor can be used, and there is no particular limitation on a semiconductor material used for the transistor 160. In terms of increasing the speed of reading data, it is preferable to use, for example, a transistor with a high switching rate such as a transistor formed using single crystal silicon.

In addition, a structure in which the capacitor 164 is not provided as illustrated in FIG. 1B can be employed.

The semiconductor device illustrated in FIG. 1A-1 utilizes a characteristic in which the potential of the gate electrode of the transistor 160 can be held, thereby writing, storing, and reading data as follows.

First, writing and holding of data will be described. First, the potential of the fourth line is set to a potential at which the transistor 162 is turned on, so that the transistor 162 is turned on. Accordingly, the potential of the third line is supplied to the gate electrode of the transistor 160 and the capacitor 164. That is, predetermined electric charge is supplied to the gate electrode of the transistor 160 (writing). Here, one of electric charges for supplying two different potentials (hereinafter, electric charge for supplying a low potential is referred to as electric charge Q_(L) and electric charge for supplying a high potential is referred to as electric charge Q_(H)) is supplied to the gate electrode of the transistor 160. Note that electric charges for supplying three or more different potentials may be used to improve storage capacity. After that, the potential of the fourth line is set to a potential at which the transistor 162 is turned off, so that the transistor 162 is turned off. Thus, the electric charge supplied to the gate electrode of the transistor 160 is held (holding).

Since the off-state current of the transistor 162 is extremely small, the electric charge in the gate electrode of the transistor 160 is held for a long time.

Next, reading of data will be described. By supplying an appropriate potential (reading potential) to the fifth line while a predetermined potential (constant potential) is supplied to the first line, the potential of the second line varies depending on the quantity of electric charge held in the gate electrode of the transistor 160. This is because in general, when the transistor 160 is an n-channel transistor, an apparent threshold voltage V_(th) _(—) _(H) in the case where Q_(H) is supplied to the gate electrode of the transistor 160 is lower than an apparent threshold voltage V_(th) _(—) _(L) in the case where Q_(L) is supplied to the gate electrode of the transistor 160. Here, an apparent threshold voltage refers to the potential of the fifth line, which is needed to turn on the transistor 160. Thus, by setting the potential of the fifth line to a potential V₀ intermediate between V_(th) _(—) _(H) and V_(th) _(—L) , the electric charge supplied to the gate electrode of the transistor 160 can be determined. For example, in the case where Q_(H) is supplied in writing, when the potential of the fifth line is set to V₀(>V_(th) _(—) _(H)), the transistor 160 is turned on. In the case where Q_(L) is supplied in writing, even when the potential of the fifth line is set to V₀ (<V_(th) _(—) _(L)), the transistor 160 remains off. Therefore, the held data can be read by recognizing the potential of the second line.

Note that in the case where memory cells are arrayed to be used, only data of a desired memory cell is needed to be read. In order that data of a predetermined memory cell is read and data of the other memory cells is not read, in the case where the transistors 160 of the memory cells are connected in parallel, fifth lines in the memory cells that are not a target for reading are supplied with a potential at which the transistors 160 are turned off regardless of the state of gate electrodes, that is, a potential lower than V_(th) _(—) _(H). On the other hand, in the case where the transistors 160 of the memory cells are connected in series, the fifth lines in the memory cells that are not a target for reading are supplied with a potential at which the transistors 160 are turned on regardless of the state of the gate electrodes, that is, a potential higher than V_(th) _(—) _(L).

Then, rewriting of data will be described. Rewriting of data is performed in a manner similar to that of the writing and holding of data. That is, the potential of the fourth line is set to a potential at which the transistor 162 is turned on, so that the transistor 162 is turned on. Accordingly, the potential of the third line (potential related to new data) is supplied to the gate electrode of the transistor 160 and the capacitor 164. After that, the potential of the fourth line is set to a potential at which the transistor 162 is turned off, so that the transistor 162 is turned off. Accordingly, electric charge related to new data is supplied to the gate electrode of the transistor 160.

In the semiconductor device according to the disclosed invention, data can be directly rewritten by another writing of data as described above. Therefore, extracting of electric charge from a floating gate with the use of a high voltage needed in a flash memory or the like is not necessary, and thus, reduction in operation speed, which is attributed to an erasing operation, can be suppressed. In other words, a high-speed operation of the semiconductor device can be realized.

Note that the drain electrode (or the source electrode) of the transistor 162 is electrically connected to the gate electrode of the transistor 160, and thereby an effect similar to that of a floating gate of a floating gate transistor which is used for a non-volatile memory element is achieved. Hereinafter, a portion where the drain electrode (or the source electrode) of the transistor 162 and the gate electrode of the transistor 160 are electrically connected to each other is called a node FG in some cases. When the transistor 162 is off, the node FG can be regarded as being embedded in an insulator and electric charge is held in the node FG. The off-state current of the transistor 162 including an oxide semiconductor is less than or equal to one hundred thousandth of the off-state current of a transistor including a silicon semiconductor or the like; thus, loss of the electric charge stored in the node FG due to leakage current of the transistor 162 is negligible. That is, with the transistor 162 including an oxide semiconductor, a non-volatile storage device which can hold data even when power is not supplied can be realized.

For example, when the off-state current of the transistor 162 is less than or equal to 10 zA (1 zA (zeptoampere) is 1×10⁻²¹ A) at room temperature (25° C.) and the capacitance value of the capacitor 164 is approximately 10 fF, data can be held for greater than or equal to 10⁴ seconds. It is needless to say that the holding time depends on the transistor characteristics and the capacitance value.

Further, the semiconductor device according to an embodiment of the disclosed invention does not have the problem of deterioration of a gate insulating layer (a tunnel insulating film), which has been a problem of a conventional floating gate transistor. That is, the problem of deterioration of a gate insulating layer due to injection of electrons into a floating gate, which has been regarded as a problem, can be solved. This means that there is no limitation on the number of times of writing operations in principle. Furthermore, a high voltage needed for writing or erasing of data in a conventional floating gate transistor is not necessary.

The components such as the transistors in the semiconductor device in FIG. 1A-1 can be regarded as including a resistor and a capacitor as illustrated in FIG. 1A-2. That is, in FIG. 1A-2, the transistor 160 and the capacitor 164 are each regarded as including a resistor and a capacitor. R1 and C1 denote the resistance value and the capacitance value of the capacitor 164, respectively. The resistance value R1 corresponds to a resistance value which depends on an insulating layer included in the capacitor 164. R2 and C2 denote the resistance value and the capacitance value of the transistor 160, respectively. The resistance value R2 corresponds to a resistance value which depends on a gate insulating layer at the time when the transistor 160 is on. The capacitance value C2 corresponds to a capacitance value of so-called gate capacitance (capacitance formed between the gate electrode and the source electrode or the drain electrode and capacitance formed between the gate electrode and the channel formation region).

An electric charge holding period (also referred to as a data holding period) is determined mainly by the off-state current of the transistor 162 under the condition that gate leakage current of the transistor 162 is sufficiently small, R1 is greater than or equal to ROS (R1≧ROS), and R2 is greater than or equal to ROS (R2≧ROS), where ROS is the resistance value (also referred to as effective resistance value) between the source electrode and the drain electrode at the time when the transistor 162 is off.

When the above condition is not met, it is difficult to sufficiently secure the holding period even if the off-state current of the transistor 162 is sufficiently small. This is because leakage current other than the off-state current of the transistor 162 (e.g., leakage current generated between the source electrode and the gate electrode) is large. Accordingly, it can be said that it is preferable that the semiconductor device disclosed in this embodiment satisfy the above relations of R1≧ROS and R2≧ROS.

It is desirable that C1 and C2 satisfy C1≧C2 (C1 is greater than or equal to C2). This is because by increasing C1, the potential of the fifth line can be effectively supplied to the node FG when the potential of the node FG is controlled by the fifth line, and thus the difference between the potentials supplied to the fifth line (e.g., a potential for reading and a potential for non-reading) can be reduced.

As described above, when the above relations are satisfied, a more favorable semiconductor device can be realized. Note that R1 and R2 are controlled by the gate insulating layer of the transistor 160 and the insulating layer of the capacitor 164. The same applies to C1 and C2. Therefore, the material, the thickness, and the like of the gate insulating layer are desirably set as appropriate to satisfy the above relation.

In the semiconductor device described in this embodiment, the node FG has an effect similar to a floating gate of a floating gate transistor in a flash memory or the like, but the node FG of this embodiment has a feature which is essentially different from that of a floating gate in a flash memory or the like.

In a flash memory, since a potential applied to a control gate is high, it is necessary to keep a proper distance between cells in order to prevent the potential from affecting a floating gate of the adjacent cell. This is one of inhibiting factors for higher integration of the semiconductor device. The factor is attributed to a basic principle of a flash memory, in which tunneling current is made to flow by applying a high electrical field.

In contrast, the semiconductor device according to this embodiment is operated by switching of a transistor including an oxide semiconductor and does not use the above-described principle of electric charge injection by tunneling current. That is, a high electrical field for electric charge injection is not necessary unlike a flash memory. Accordingly, it is not necessary to consider an influence of a high electrical field from a control gate on the adjacent cell, which facilitates higher integration.

In addition, it is also advantageous that a high electric field is unnecessary and a large peripheral circuit (such as a step-up circuit) is unnecessary as compared to a flash memory. For example, the highest voltage applied to the memory cell according to this embodiment (the difference between the highest potential and the lowest potential applied to terminals of the memory cell at the same time) can be less than or equal to 5 V, preferably less than or equal to 3 V in each memory cell in the case where two levels (one bit) of data are written.

In the case where the relative permittivity εr1 of the insulating layer included in the capacitor 164 is made different from the relative permittivity εr2 of the insulating layer included in the transistor 160, C1 can easily be made greater than or equal to C2 (C1≧C2) while S1 which is the area of the insulating layer included in the capacitor 164 and S2 which is the area of the insulating layer forming gate capacitance of the transistor 160 satisfy a relation where 2·S2 greater than or equal to S1 (2·S2≧S1), desirably S2 is greater than or equal to S1 (S2≧S1). That is, it is easy to satisfy C1≧C2 while the area of the insulating layer included in the capacitor 164 is made small. Specifically, for example, a film formed of a high-k material such as hafnium oxide or a stack of a film formed of a high-k material such as hafnium oxide and a film formed of an oxide semiconductor is used for the insulating layer included in the capacitor 164 so that εr1 can be greater than or equal to 10, preferably greater than or equal to 15, and silicon oxide is used for the insulating layer forming the gate capacitance of the transistor 160 so that εr2 can be 3 to 4.

Combination of such structures enables higher integration of the semiconductor device according to the disclosed invention.

Note that in addition to the increase in the degree of integration, a multilevel technique can be employed in order to increase the storage capacity of the semiconductor device. For example, three or more levels of data are written into one memory cell, whereby the storage capacity can be increased as compared to that in the case where two-level (one-bit) data is written. The multilevel technique can be achieved by, for example, supplying electric charge Q for supplying another level of potential to the gate electrode of the transistor 160, in addition to electric charge Q_(L), for supplying a low potential and electric charge Q_(H) for supplying a high potential. In this case, sufficient storage capacity can be ensured even in a circuit structure with a relatively large scale (e.g., 15 F² to 50 F²; F is the minimum feature size).

<Basic Circuit 2>

Next, a more specific circuit structure and its operation will be described with reference to FIG. 2. FIG. 2 illustrates a structure in which two memory cells are arranged in the row direction. A structure of a memory cell 170 illustrated in FIG. 2 is similar to that in FIG. 1A-1. That is, the first line in FIG. 1A-1 corresponds to a source line SL in FIG. 2. The fourth line in FIG. 1A-1 corresponds to a write word line OSG in FIG. 2. The fifth line in FIG. 1A-1 corresponds to a write-read word line C in FIG. 2. Both the second line and the third line in FIG. 1A-1 correspond to a bit line BL in FIG. 2. In FIG. 2, the source line SL is used for the memory cells in a plurality of columns (e.g., two columns in FIG. 2). The number of columns for which one source line SL is used is not limited to 2, and one source line SL may be used for memory cells in three or more columns. In addition, one source line SL may be used for memory cells in a plurality of rows or for memory cells in a plurality of columns and a plurality of rows.

In FIG. 2, the source line SL is connected to a source line switching circuit 194. Here, the source line switching circuit 194 is connected to a write-enable bar signal line WEB as well as to the source line SL. The write-enable bar signal line WEB transmits, to the source line switching circuit 194, a signal for switching a signal path by the control of a writing period or a reading period.

Writing, holding, and reading of data are performed in a manner similar to that in the case of FIGS. 1A-1, 1A-2, and 1B. Here, as an example, the case where either a power supply potential VDD or a reference potential GND is supplied to the node FG will be described. Note that data “1” is held when the power supply potential VDD is supplied to the node FG whereas data “0” is held when the reference potential GND is supplied to the node FG.

A specific writing operation is as follows. First, a potential of the write-read word line C connected to the memory cell 170 is set to GND and a potential of the write word line OSG connected to the memory cell 170 is set to VDD, whereby the memory cell 170 is selected. Accordingly, the potential of the bit line BL is supplied to the node FG of the selected memory cell 170.

Here, in the case where the power supply potential VDD is supplied to the node FG (i.e., in the case where data “1” is held), a potential at which the transistor 160 is turned on is supplied to the gate electrode of the transistor 160. In that case, the potential of the source line SL needs to be set to the power potential VDD in order to suppress decrease in potential written into the node FG due to current flowing in the bit line BL and the source line SL.

In that case, with a signal transmitted from the write-enable bar signal line WEB, a signal path in the source line switching circuit 194 is switched. That is, a signal with which a writing operation can be performed is input to the write-enable bar signal line WEB and the signal path in the source line switching circuit 194 is switched, whereby the power supply potential VDD is supplied to the source line SL.

The operation is characterized in that the potential of the source line SL is set to the power supply potential VDD when data is written. This can suppress generation of current flowing in the bit line BL and the source line SL even when the potential at which the transistor 160 is turned on is supplied to the node FG.

Note that in the case where data “0” is written into the memory cell 170, GND is supplied to the bit line BL; in the case where data “1” is written into the memory cell 170, the potential of the bit line BL is set to VDD. Note that when data “1” is written into the memory cell, VDD+Vth_(—) _(OS) is supplied to the bit line BL so that the potential of the node FG is not reduced from VDD by the threshold voltage of the transistor 162 (Vth_(—) _(OS) ). Alternatively, the write word line OSG may have a high potential (VH: a potential higher than the sum of the potential of the bit line BL and Vth_(—) _(OS) ).

Data is held in such a manner that the write-read word line C and the write word line OSG are each set to a potential (VL) lower than GND by VDD.

When the potential of the write-read word line C is decreased to VL from GND, the potential of the node FG is decreased by GND−VL (=VDD) by capacitive coupling with the capacitor 164. Accordingly, when VDD corresponding to data “1” has been supplied to the node FG, the potential of the node FG is decreased by VDD to be GND, whereas when GND corresponding to data “0” has been supplied to the node FG; the potential of the node FG is decreased by VDD to be GND−VDD (=VL).

Since VL has been supplied to the write word line OSG, the transistor 162 is turned off regardless of whether data “1” or data “0” has been written. Since the off-state current of the transistor 162 is significantly small, the electric charge of the gate electrode of the transistor 160 is held for a long time.

Data is read by setting the potential of the write-read word line C to GND and the potential of the write word line OSG to VL.

When the potential of the write-read word line C is increased to GND from VL, the potential of the node FG is increased by GND−VL (=VDD) by capacitive coupling with the capacitor 164. Accordingly, when data “1” has been written in the node FG, the potential of the node FG is increased by VDD to be VDD. On the other hand, when data “0” has been written in the node FG; the potential of the node FG is increased by VDD to be VL+VDD (=GND).

In the reading operation, with a signal transmitted from the write-enable bar signal line WEB, the signal path in the source line switching circuit 194 is switched. Through the above operation, GND is supplied to the source line SL.

By the above reading operation, if data “1” has been written into the memory cell 170, the transistor 160 is turned on and the potential of the bit line BL is decreased. If data “0” has been written, the transistor 160 is turned off and the potential of the bit line BL is maintained at the level at the beginning of reading or is increased.

As described above, in the semiconductor device having the circuit structure illustrated in FIG. 2, the area of the memory cell array can be reduced by sharing the source line SL by memory cells in a plurality of columns. Accordingly, the die size can be reduced. Moreover, the reduction in the die size allows cost reduction in manufacturing the semiconductor device or improvement in yield.

Application Example

Next, an example of a semiconductor device in which the above-described basic circuits are arranged in a matrix will be described with reference to FIG. 3 and FIG. 4.

FIG. 3 illustrates an example of a circuit diagram of a semiconductor device including m×n memory cells 170. The structure of the memory cell 170 in FIG. 3 is similar to that in FIG. 1A-1. That is, the first line in FIG. 1A-1 corresponds to a source line SL in FIG. 3. The fourth line in FIG. 1A-1 corresponds to a write word line OSG in FIG. 3. The fifth line in FIG. 1A-1 corresponds to a write-read word line C in FIG. 3. Note that both the second line and the third line in FIG. 1A-1 correspond to a bit line BL in FIG. 3. In FIG. 3, the source line SL is used for memory cells in a plurality of columns (e.g., n columns in FIG. 3). The number of columns for which one source line SL is used is not limited to that in FIG. 3. For example, source lines SL whose number is greater than or equal to 2 and less than n may be provided for memory cells in n columns.

The semiconductor device according to this embodiment includes m (m is an integer of greater than or equal to 2) write word lines OSG, m write-read word lines C, n (n is an integer of greater than or equal to 2) bit lines BL, a source line SL, a memory cell array having the memory cells 170 arranged in a matrix of m (rows)×n (columns), a first driver circuit 190, a second driver circuit 192, and a source line switching circuit 194.

To the first driver circuit 190, a chip-enable bar signal line CEB, a write-enable bar signal line WEB, a read-enable bar signal line REB, an address selection signal line A, a data input signal line DIN, a data output signal line DOUT, and the bit line BL are connected.

A signal with which the semiconductor device is selected or becomes active is input to the chip-enable bar signal line CEB. Note that the chip-enable bar signal line CEB is not always necessary. In addition, a signal related to a writing operation is input to the write-enable bar signal line WEB, and a signal related to a reading operation is input to the read-enable bar signal line REB. In this embodiment, each of the chip-enable bar signal line CEB, the write-enable bar signal line WEB, and the read-enable bar signal line REB becomes active when a low potential is input thereto; but the present invention can also be embodied in a different manner, and a circuit structure in which the above signal lines become active when a high potential is input thereto may be employed.

The data input signal line DIN corresponds to a terminal to which data to be written into the memory cell 170 is input. The data output signal line DOUT corresponds to a terminal from which data that has been written into the memory cell 170 is output. One or more data input signal lines DIN and data output signal lines DOUT are provided, and the number of these lines depends on the circuit structure of the first driver circuit 190. Alternatively, a data input output signal line serving as both the data input signal line DIN and the data output signal line DOUT may be employed. The data input output signal line has both a function of input of data to be written into the memory cell 170 and a function of output of data that has been written into the memory cell 170.

The address selection signal line A corresponds to a terminal to which a signal for selecting an address of the memory cell 170 is input. One or more address selection signal lines A are provided, and the number of the address selection signal lines A depends on the number of rows and the number of columns of the memory cells, the circuit structure of the first driver circuit 190, the circuit structure of the second driver circuit 192, and the like.

A data latch circuit, a page buffer, and the like in which input data and output data are temporarily stored may be provided in the first driver circuit 190.

To the second driver circuit 192, the chip-enable bar signal line CEB, the write-enable bar signal line WEB, the read-enable bar signal line REB, the address selection signal line A, the write word line OSG and the write-read word line C are connected. In order to prevent a potential that will be written into the node FG of the memory cell 170 from being decreased by the threshold voltage (V_(th) _(—) _(OS)) of the transistor 162, the potential of the write word line OSG needs to be set high (VH: a potential higher than the sum of the potential of the bit line BL and V_(th) _(—) _(OS)). In order to generate the potential VII, a step-up circuit may be provided in the second driver circuit 192.

To the source line switching circuit 194, the write-enable bar signal line WEB and the source line SL are connected. The source line switching circuit 194 has a function of switching the potential of the source line SL depending on the potential of the write-enable bar signal line WEB.

FIG. 4 is an example of a timing chart of the semiconductor device in FIG. 3. CEB, WEB, and the like in the timing chart denote the lines to which the potentials in the timing chart are supplied. A plurality of lines having a similar function are distinguished by “_(—)1”, “_(—)2”, and the like added to the end of their names.

The timing chart in FIG. 4 shows the relation among the potentials of the lines in the case where data “1” is written into the memory cell in the first row and the first column, data “0” is written into the memory cell in the first row and the n-th column, data “0” is written into the memory cell in the m-th row and the first column, and data “1” is written into the memory cell in the m-th row and the n-th column, and then the written data in all the above memory cells is read out.

In a writing period, the chip-enable bar signal line CEB and the write-enable bar signal line WEB are set to a low potential and an address of the memory cell to which data is written is specified by a signal from the address selection signal line A, whereby a state in which writing can be performed is made. Then, data is input from the data input signal line DIN, so that writing is performed. Data to be written is output from the first driver circuit 190 to the bit line BL. High potentials are output from the second driver circuit 192 to the write word line OSG in a selected row and the write-read word line C in a selected row. A high potential is output from the source line switching circuit 194 to the source line SL.

In the writing period, data to be written is output to the bit line BL from the first driver circuit 190 in accordance with the timing of row selection. The bit line BL in the case of writing of data “1” has a high potential, whereas the bit line BL in the case of writing of data “0” has a low potential. Note that a signal input period of the bit line BL is set so as to be longer than that of the write word line OSG in a selected row and the write-read word line C in a selected row for the following reason: if the signal input period of the bit line BL is short, incorrect writing of data to the memory cell might be caused.

In a reading period, the chip-enable bar signal line CEB and the read-enable bar signal line REB are set to a low potential and an address of the memory cell from which data is read is specified by a signal from the address selection signal line A, whereby a state in which reading can be performed is made. Data which has been read from the memory cell via the bit line BL is output from the first driver circuit 190 to the data output signal line DOUT. A high potential is output from the second driver circuit 192 to the write-read word line C in a selected row. The write word line OSG has a low potential regardless of a selected state or a non-selected state.

In the reading period, a potential corresponding to the data which has been written into the memory cell is output to the bit line BL in accordance with the timing of row selection. The bit line BL has a low potential when data “1” has been written into the memory cell, whereas the bit line BL has a high potential when data “0” has been written into the memory cell.

In a standby and data holding period, the chip-enable bar signal line CEB is set to a high potential (e.g., a power supply potential), so that the memory cell is in a non-selected state or a disable state. In this case, neither writing nor reading is performed; thus, the write-enable bar signal line WEB, the read-enable bar signal line REB, and the like may have either a high potential or a low potential.

As described above, in the semiconductor device having the circuit structure illustrated in FIG. 3, the area of the memory cell array can be reduced by sharing the source line SL by memory cells in a plurality of columns. Accordingly, the die size can be reduced. Moreover, the reduction in the die size allows cost reduction in manufacturing the semiconductor device or improvement in yield.

The structures, the methods, and the like described in this embodiment can be combined as appropriate with any of the structures, the methods, and the like described in the other embodiments.

Embodiment 2

In this embodiment, a structure and a manufacturing method of a semiconductor device according to an embodiment of the disclosed invention will be described with reference to FIGS. 5A and 5B, FIGS. 6A to 6G, FIGS. 7A to 7E, FIGS. 8A to 8D, FIGS. 9A to 9D, and FIGS. 10A to 10C.

<Cross-Sectional Structure and Planar Structure of Semiconductor Device>

FIGS. 5A and 5B illustrate an example of a structure of a semiconductor device. FIG. 5A is a cross-sectional view of the semiconductor device, and FIG. 5B is a plan view of the semiconductor device. FIG. 5A corresponds to a cross section along line A1-A2 and line B1-B2 in FIG. 5B. The semiconductor device illustrated in FIGS. 5A and 5B includes a transistor 160 including a first semiconductor material in a lower portion, and a transistor 162 including a second semiconductor material in an upper portion. Here, the first semiconductor material and the second semiconductor material are preferably different materials. For example, a semiconductor material other than an oxide semiconductor can be used as the first semiconductor material, and an oxide semiconductor can be used as the second semiconductor material. The semiconductor material other than an oxide semiconductor can be, for example, silicon, germanium, silicon germanium, silicon carbide, gallium arsenide, or the like and is preferably a single crystal semiconductor. Alternatively, an organic semiconductor material or the like may be used. A transistor including such a semiconductor material other than an oxide semiconductor can operate at high speed easily. On the other hand, a transistor including an oxide semiconductor can hold electric charge for a long time owing to its characteristics. The semiconductor device in FIGS. 5A and 5B can be used as a memory cell.

Since the technical nature of the disclosed invention is to use a semiconductor material with which off-state current can be sufficiently decreased, such as an oxide semiconductor, in the transistor 162 so that data can be held, it is not necessary to limit a specific structure of the semiconductor device, such as a material of the semiconductor device or a structure of the semiconductor device, to those described here.

The transistor 160 in FIGS. 5A and 5B includes a channel formation region 134 provided in a semiconductor layer over a semiconductor substrate 500, impurity regions 132 (also referred to as a source region and a drain region) with the channel formation region 134 provided therebetween, a gate insulating layer 122 a provided over the channel formation region 134, and a gate electrode 128 a provided over the gate insulating layer 122 a so as to overlap with the channel formation region 134. Note that a transistor whose source electrode and drain electrode are not illustrated in a drawing is referred to as a transistor for the sake of convenience in some cases. Further, in such a case, in description of a connection of a transistor, a source region and a source electrode are collectively referred to as a “source electrode,” and a drain region and a drain electrode are collectively referred to as a “drain electrode”. That is, in this specification, the term “source electrode” may include a source region. Also, the teen “drain electrode” may include a drain region.

Further, a conductive layer 128 b is connected to an impurity region 126 provided in the semiconductor layer over the semiconductor substrate 500. Here, the conductive layer 128 b functions as a source electrode or a drain electrode of the transistor 160. In addition, an impurity region 130 is provided between the impurity region 132 and the impurity region 126. Further, insulating layers 136, 138, and 140 are provided so as to cover the transistor 160. Note that for higher integration, it is preferable that, as in FIGS. 5A and 5B, the transistor 160 does not have a sidewall insulating layer. However, when importance is put on the characteristics of the transistor 160, sidewall insulating layers may be provided on side surfaces of the gate electrode 128 a, and the impurity region 132 may include a region with a different impurity concentration.

The transistor 162 in FIGS. 5A and 5B includes an oxide semiconductor layer 144 which is provided over the insulating layer 140 and the like; a source electrode (or a drain electrode) 142 a and a drain electrode (or a source electrode) 142 b which are electrically connected to the oxide semiconductor layer 144; a gate insulating layer 146 which covers the oxide semiconductor layer 144, the source electrode 142 a, and the drain electrode 142 b; and a gate electrode 148 a which is provided over the gate insulating layer 146 so as to overlap with the oxide semiconductor layer 144.

Here, the oxide semiconductor layer 144 is preferably a highly purified oxide semiconductor layer obtained by sufficiently removing impurities such as hydrogen or sufficiently supplying oxygen. Specifically, the hydrogen concentration of the oxide semiconductor layer 144 is less than or equal to 5×10¹⁹ atoms/cm³, preferably less than or equal to 5×10¹⁸ atoms/cm³, more preferably less than or equal to 5×10¹⁷ atoms/cm³. Note that the hydrogen concentration of the oxide semiconductor layer 144 is measured by secondary ion mass spectrometry (SIMS). In the oxide semiconductor layer 144 which is highly purified by sufficiently reducing the hydrogen concentration and in which defect levels in an energy gap due to oxygen deficiency are reduced by sufficiently supplying oxygen, the carrier concentration is less than 1×10¹²/cm³, preferably less than 1×10¹¹/cm³, more preferably less than 1.45×10¹⁰/cm³. For example, the off-state current (here, current per micrometer (μm) of channel width) at room temperature (25° C.) is less than or equal to 100 zA (1 zA (zeptoampere) is 1×10⁻²¹ A), preferably less than or equal to 10 zA. In this manner, by using an oxide semiconductor which is made to be an i-type (intrinsic) oxide semiconductor or a substantially i-type oxide semiconductor, the transistor 162 which has extremely favorable off-state current characteristics can be obtained.

Note that although the transistor 162 in FIGS. 5A and 5B includes the oxide semiconductor layer 144 which is processed into an island shape in order to suppress leakage current between elements which is caused due to miniaturization, the oxide semiconductor layer 144 which is not processed into an island shape may be employed. In the case where the oxide semiconductor layer is not processed into an island shape, contamination of the oxide semiconductor layer 144 due to etching in the processing can be prevented.

A capacitor 164 in FIGS. 5A and 5B includes the drain electrode 142 a, the gate insulating layer 146, and a conductive layer 148 b. That is to say, the drain electrode 142 b functions as one electrode of the capacitor 164, and the conductive layer 148 b functions as the other electrode of the capacitor 164. With such a structure, sufficient capacitance can be secured. Further, in the capacitor 164, insulating properties between the drain electrode 142 b and the conductive layer 148 b can be adequately secured by stacking the oxide semiconductor layer 144 and the gate insulating layer 146. Further alternatively, the capacitor 164 may be omitted in the case where a capacitor is not needed.

In this embodiment, the transistor 162 and the capacitor 164 are provided so as to overlap with the transistor 160 at least partly. By employing such a planar layout, higher integration can be realized. For example, given that the minimum feature size is F, the area of a memory cell can be 15 F² to 25 F².

An insulating layer 150 is provided over the transistor 162 and the capacitor 164. A wiring 154 is provided in an opening formed in the gate insulating layer 146 and the insulating layer 150. The wiring 154 is a wiring which connects one memory cell to another memory cell and corresponds to the bit line BL in the circuit diagram in FIG. 2. The wiring 154 is electrically connected to the impurity region 126 through the source electrode 142 a and the conductive layer 128 b. The above structure allows reduction in the number of wirings as compared to a structure in which the source region or the drain region in the transistor 160 and the source electrode 142 a in the transistor 162 are connected to different wirings. Thus, the integration degree of the semiconductor device can be increased.

Since the conductive layer 128 b is provided, a position where the impurity region 126 and the source electrode 142 a are connected and a position where the source electrode 142 a and the wiring 154 are connected can overlap with each other. With such a planar layout, the element area can be prevented from increasing owing to contact regions. In other words, the degree of integration of the semiconductor device can be increased.

<Manufacturing Method of SOI Substrate>

Next, an example of a method for manufacturing an SOI substrate used for manufacturing the semiconductor device will be described with reference to FIGS. 6A to 6G.

First, the semiconductor substrate 500 is prepared as a base substrate (see FIG. 6A). As the semiconductor substrate 500, a semiconductor substrate such as a single crystal silicon substrate or a single crystal germanium substrate can be used. In addition, as the semiconductor substrate, a solar grade silicon (SOG-Si) substrate or the like may be used. Alternatively, a polycrystalline semiconductor substrate may be used. In the case of using a SOG-Si substrate, a polycrystalline semiconductor substrate, or the like, manufacturing cost can be reduced as compared to the case of using a single crystal silicon substrate or the like.

In stead of the semiconductor substrate 500, a variety of glass substrates used in electronic industries, such as a substrate of aluminosilicate glass, aluminoborosilicate glass, or barium borosilicate glass, a quartz substrate, a ceramic substrate, or a sapphire substrate can be used. Further, a ceramic substrate which contains silicon nitride and aluminum nitride as its main components and whose coefficient of thermal expansion is close to that of silicon may be used.

A surface of the semiconductor substrate 500 is preferably cleaned in advance. Specifically, the semiconductor substrate 500 is subjected to cleaning with a hydrochloric acid/hydrogen peroxide mixture (RPM), a sulfuric acid/hydrogen peroxide mixture (SPM), an ammonium hydrogen peroxide mixture (APM), diluted hydrofluoric acid (DHF), or the like.

Next, a bond substrate is prepared. Here, a single crystal semiconductor substrate 510 is used as the bond substrate (see FIG. 6B). Note that although a substrate whose crystallinity is single crystal is used as the bond substrate here, the crystallinity of the bond substrate is not necessarily limited to single crystal.

For example, as the single crystal semiconductor substrate 510, a single crystal semiconductor substrate formed using a Group 14 element, such as a single crystal silicon substrate, a single crystal germanium substrate, or a single crystal silicon germanium substrate, can be used. Further, a compound semiconductor substrate using gallium arsenide, indium phosphide, or the like can be used. Typical examples of commercially available silicon substrates are circular silicon substrates which are 5 inches (125 mm) in diameter, 6 inches (150 mm) in diameter, 8 inches (200 mm) in diameter, 12 inches (300 mm) in diameter, and 16 inches (400 mm) in diameter. Note that the shape of the single crystal semiconductor substrate 510 is not limited to circular, and the single crystal semiconductor substrate 510 may be a substrate which is processed into, for example, a rectangular shape or the like. Further, the single crystal semiconductor substrate 510 can be formed by a Czochralski (CZ) method or a Floating Zone (FZ) method.

An oxide film 512 is formed on a surface of the single crystal semiconductor substrate 510 (see FIG. 6C). In terms of removal of contamination, it is preferable that the surface of the single crystal semiconductor substrate 510 be cleaned with a hydrochloric acid/hydrogen peroxide mixture (HPM), a sulfuric acid/hydrogen peroxide mixture (SPM), an ammonium hydrogen peroxide mixture (APM), diluted hydrofluoric acid (DHF), FPM (a mixed solution of hydrofluoric acid, hydrogen peroxide water, and pure water), or the like before the formation of the oxide film 512. Alternatively, diluted hydrofluoric acid and ozone water may be discharged alternately to clean the surface of the single crystal semiconductor substrate 510.

The oxide film 512 can be formed with, for example, a single layer or a stacked layer of a silicon oxide film, a silicon oxynitride film, and the like. As a method for forming the oxide film 512, a thermal oxidation method, a CVD method, a sputtering method, or the like can be used. When the oxide film 512 is formed by a CVD method, a silicon oxide film is preferably formed using organosilane such as tetraethoxysilane (abbreviation: TEOS) (chemical formula: Si(OC₂H₅)₄), so that favorable bonding can be achieved.

In this embodiment, the oxide film 512 (here, a SiO_(x) film) is formed by performing thermal oxidation treatment on the single crystal semiconductor substrate 510. The thermal oxidation treatment is preferably performed in an oxidizing atmosphere to which halogen is added.

For example, thermal oxidation treatment is performed on the single crystal semiconductor substrate 510 in an oxidizing atmosphere to which chlorine (Cl) is added, whereby the oxide film 512 can be formed through chlorine oxidation. In this case, the oxide film 512 is a film containing chlorine atoms. By such chlorine oxidation, heavy metal (e.g., Fe, Cr, Ni, or Mo) that is an extrinsic impurity is trapped and chloride of the metal is formed and then removed to the outside; thus, contamination of the single crystal semiconductor substrate 510 can be reduced.

Note that halogen atoms contained in the oxide film 512 are not limited to chlorine atoms. Fluorine atoms may be contained in the oxide film 512. As a method for fluorine oxidation of the surface of the single crystal semiconductor substrate 510, a method in which the single crystal semiconductor substrate 510 is soaked in an HF solution and then subjected to thermal oxidation treatment in an oxidizing atmosphere, a method in which thermal oxidation treatment is performed in an oxidizing atmosphere to which NF₃ is added, or the like can be used.

Next, ions are accelerated by an electric field and the single crystal semiconductor substrate 510 is irradiated with the ions, and the ions are added to the single crystal semiconductor substrate 510, whereby an embrittled region 514 where the crystal structure is damaged is formed in the single crystal semiconductor substrate 510 at a predetermined depth (see FIG. 6D).

The depth at which the embrittled region 514 is formed can be adjusted by the kinetic energy, mass, electric charge, or incidence angle of the ions, or the like. The embrittled region 514 is formed at approximately the same depth as the average penetration depth of the ions. Therefore, the thickness of a single crystal semiconductor layer to be separated from the single crystal semiconductor substrate 510 can be adjusted by the depth at which the ions are added. For example, the average penetration depth may be controlled such that the thickness of the single crystal semiconductor layer is approximately greater than or equal to 10 nm and less than or equal to 500 nm, preferably, greater than or equal to 50 nm and less than or equal to 200 nm.

The above ion irradiation treatment can be performed with the use of an ion doping apparatus or an ion implantation apparatus. As a typical example of the ion doping apparatus, there is a non-mass-separation type apparatus in which plasma excitation of a process gas is performed and an object to be processed is irradiated with all kinds of ion species generated. In this apparatus, the object to be processed is irradiated with ion species of plasma without mass separation. In contrast, an ion implantation apparatus is a mass-separation type apparatus. In the ion implantation apparatus, mass separation of ion species of plasma is performed and the object to be processed is irradiated with ion species having predetermined masses.

In this embodiment, an example is described in which an ion doping apparatus is used to add hydrogen to the single crystal semiconductor substrate 510. A gas containing hydrogen is used as a source gas. As for ions used for the irradiation, the proportion of H₃ ⁺ is preferably set high. Specifically, it is preferable that the proportion of H₃ ⁺ be set to greater than or equal to 50% (more preferably, greater than or equal to 80%) with respect to the total amount of H⁺, H₂ ⁺, and H₃ ⁺. With a high proportion of H₃ ⁺, the efficiency of ion irradiation can be improved.

Note that ions to be added are not limited to ions of hydrogen. Ions of helium or the like may be added. Further, ions to be added are not limited to one kind of ions, and plural kinds of ions may be added. For example, in the case of performing irradiation with hydrogen and helium concurrently using an ion doping apparatus, the number of steps can be reduced as compared to the case of performing irradiation with hydrogen and helium in different steps, and surface roughness of the single crystal semiconductor layer to be formed later can be suppressed.

Note that heavy metal might be added when the embrittled region 514 is formed with the ion doping apparatus; however, by performing the ion irradiation through the oxide film 512 containing halogen atoms, contamination of the single crystal semiconductor substrate 510 due to the heavy metal can be prevented.

Then, the semiconductor substrate 500 and the single crystal semiconductor substrate 510 are disposed to face each other and are made to be closely attached to each other with the oxide film 512 therebetween. Thus, the semiconductor substrate 500 and the single crystal semiconductor substrate 510 can be bonded to each other (see FIG. 6E). Note that an oxide film or a nitride film may be formed on the surface of the semiconductor substrate 500 to which the single crystal semiconductor substrate 510 is attached.

When bonding is performed, it is preferable that a pressure of greater than or equal to 0.001 N/cm² and less than or equal to 100 N/cm², e.g., a pressure of greater than or equal to 1 N/cm² and less than or equal to 20 N/cm², be applied to one part of the semiconductor substrate 500 or one part of the single crystal semiconductor substrate 510. When the bonding surfaces are made close to each other and disposed in close contact with each other by applying a pressure, a bonding between the semiconductor substrate 500 and the oxide film 512 is generated at the part where the close contact is made, and the bonding spontaneously spreads to almost the entire area. This bonding is performed under the action of the Van der Waals force or hydrogen bonding and can be performed at room temperature.

Note that before the single crystal semiconductor substrate 510 and the semiconductor substrate 500 are bonded to each other, surfaces to be bonded to each other are preferably subjected to surface treatment. Surface treatment can improve the bonding strength at the interface between the single crystal semiconductor substrate 510 and the semiconductor substrate 500.

As the surface treatment, wet treatment, dry treatment, or a combination of wet treatment and dry treatment can be used. Alternatively, wet treatment may be performed in combination with different wet treatment or dry treatment may be performed in combination with different dry treatment.

Note that heat treatment for increasing the bonding strength may be performed after bonding. This heat treatment is performed at a temperature at which separation at the embrittled region 514 does not occur (for example, a temperature of greater than or equal to room temperature and less than 400° C.). Alternatively, bonding of the semiconductor substrate 500 and the oxide film 512 may be performed while heating them at a temperature in the above range. The heat treatment can be performed using a diffusion furnace, a heating furnace such as a resistance heating furnace, a rapid thermal annealing (RTA) apparatus, a microwave heating apparatus, or the like. The above temperature condition is merely an example, and an embodiment of the disclosed invention should not be construed as being limited to this example.

Next, heat treatment is performed for separation of the single crystal semiconductor substrate 510 at the embrittled region 514, whereby a single crystal semiconductor layer 516 is formed over the semiconductor substrate 500 with the oxide film 512 interposed therebetween (FIG. 6F).

Note that the temperature for the heat treatment in the separation is desirably as low as possible. This is because as the temperature in the separation is low, generation of roughness on the surface of the single crystal semiconductor layer 516 can be suppressed. Specifically, the temperature for the heat treatment in the separation may be greater than or equal to 300° C. and less than or equal to 600° C. and the heat treatment is more effective when the temperature is greater than or equal to 400° C. and less than or equal to 500° C.

Note that after the single crystal semiconductor substrate 510 is separated, the single crystal semiconductor layer 516 may be subjected to heat treatment at greater than or equal to 500° C. so that concentration of hydrogen remaining in the single crystal semiconductor layer 516 can be reduced.

Next, a surface of the single crystal semiconductor layer 516 is irradiated with laser light, whereby a single crystal semiconductor layer 518 in which the flatness of the surface is improved and the number of defects is reduced is formed (see FIG. 6G). Note that instead of the laser light irradiation treatment, heat treatment may be performed.

Although the laser light irradiation treatment is performed immediately after the heat treatment for separation of the single crystal semiconductor layer 516 in this embodiment, an embodiment of the present invention is not construed as being limited to this. Etching treatment may be performed after the heat treatment for separation of the single crystal semiconductor layer 516, to remove a region where there are many defects on the surface of the single crystal semiconductor layer 516, and then the laser light irradiation treatment may be performed. Alternatively, after the surface flatness of the single crystal semiconductor layer 516 is improved, the laser light irradiation treatment may be performed. Note that the etching treatment may be either wet etching or dry etching. Further, in this embodiment, a step of reducing the thickness of the single crystal semiconductor layer 516 may be performed after the laser light irradiation. In order to reduce the thickness of the single crystal semiconductor layer 516, any one of or both dry etching and wet etching may be employed.

Through the above steps, an SOI substrate having the single crystal semiconductor layer 518 with favorable characteristics can be obtained (see FIG. 6G).

<Method for Manufacturing Semiconductor Device>

Next, a method for manufacturing a semiconductor device using the SOI substrate will be described with reference to FIGS. 7A to 7E, FIGS. 8A to 8D, FIGS. 9A to 9D, and FIGS. 10A to 10C.

<Method for Manufacturing Transistor in Lower Portion>

First, a manufacturing method of the transistor 160 in the lower portion will be described with reference to FIGS. 7A to 7E and FIGS. 8A to 8D. Note that FIGS. 7A to 7E and FIGS. 8A to 8D illustrate part of the SOI substrate formed by the method illustrated in FIGS. 6A to 6G and are cross-sectional views illustrating the steps for manufacturing the transistor in the lower portion illustrated in FIG. 5A.

First, the single crystal semiconductor layer 518 is processed into an island shape, so that a semiconductor layer 120 is formed (see FIG. 7A). Note that before or after this step, an impurity element imparting n-type conductivity or an impurity element imparting p-type conductivity may be added to the semiconductor layer in order to control the threshold voltage of the transistor. In the case where silicon is used as the semiconductor, phosphorus, arsenic, or the like can be used as the impurity element imparting n-type conductivity. On the other hand, boron, aluminum, gallium, or the like can be used as the impurity element imparting p-type conductivity.

Next, an insulating layer 122 is formed so as to cover the semiconductor layer 120 (see FIG. 7B). The insulating layer 122 is to be a gate insulating layer later. The insulating layer 122 can be formed, for example, by performing heat treatment (thermal oxidation treatment, thermal nitridation treatment, or the like) on a surface of the semiconductor layer 120. Instead of the heat treatment, high-density plasma treatment may be employed. The high-density plasma treatment can be performed using, for example, a mixed gas of any of a rare gas such as He, Ar, Kr, or Xe, oxygen, nitrogen oxide, ammonia, nitrogen, and hydrogen. Needless to say, the insulating layer may be formed using a CVD method, a sputtering method, or the like. The insulating layer 122 preferably has a single-layer structure or a layered structure including any of silicon oxide, silicon oxynitride, silicon nitride, hafnium oxide, aluminum oxide, tantalum oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate to which nitrogen is added (HfSi_(x)O_(y)N_(z) (x>0, y>0, z>0)), hafnium aluminate to which nitrogen is added (HfAl_(x)O_(y)N_(z) (x>0, y>0, z>0)), and the like. The thickness of the insulating layer 122 may be, for example, greater than or equal to 1 nm and less than or equal to 100 nm, preferably greater than or equal to 10 nm and less than or equal to 50 nm In this embodiment, a single-layer insulating layer containing silicon oxide is formed by a plasma CVD method.

Next, a mask 124 is formed over the insulating layer 122 and the impurity element imparting n-type conductivity is added to the semiconductor layer 120, so that the impurity region 126 is formed (see FIG. 7C). Note that the mask 124 is removed after the impurity element is added.

Next, a mask is formed over the insulating layer 122 and a region of the insulating layer 122 that overlaps with the impurity region 126 is partly removed, so that the gate insulating layer 122 a is formed (see FIG. 7D). Part of the insulating layer 122 can be removed by etching treatment such as wet etching or dry etching.

Next, a conductive layer for forming a gate electrode (including a wiring formed using the same layer as the gate electrode) is formed over the gate insulating layer 122 a and is processed, so that the gate electrode 128 a and the conductive layer 128 b are formed (see FIG. 7E).

The conductive layer used for the gate electrode 128 a and the conductive layer 128 b can be formed using a metal material such as aluminum, copper, titanium, tantalum, or tungsten. Further, the conductive layer may be formed using a semiconductor material such as polycrystalline silicon. There is no particular limitation on the method for forming the conductive layer, and a variety of film formation methods such as an evaporation method, a CVD method, a sputtering method, or a spin coating method can be employed. The conductive layer can be processed by etching using a resist mask.

Next, an impurity element imparting one conductivity type is added to the semiconductor layer with the use of the gate electrode 128 a and the conductive layer 128 b as masks, so that the channel formation region 134, the impurity regions 132, and the impurity region 130 are formed (see FIG. 8A). Here, in the case of forming an n-channel transistor, an impurity element such as phosphorus (P) or arsenic (As) is added. Here, the concentration of the impurity element to be added can be set as appropriate. In addition, after the impurity element is added, heat treatment for activation is performed. Here, the concentration of the impurity element in the impurity region is increased in the following order: the impurity region 126, the impurity region 132, and the impurity region 130.

Next, the insulating layer 136, the insulating layer 138, and the insulating layer 140 are formed so as to cover the gate insulating layer 122 a, the gate electrode 128 a, and the conductive layer 128 b (see FIG. 8B).

The insulating layer 136, the insulating layer 138, and the insulating layer 140 can be formed using a material including an inorganic insulating material such as silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, or aluminum oxide. In particular, the insulating layer 136, the insulating layer 138, and the insulating layer 140 are preferably formed using a low permittivity (low-k) material, because capacitance due to an overlapping electrode or wiring can be sufficiently reduced. Note that the insulating layer 136, the insulating layer 138, and the insulating layer 140 may be porous insulating layers formed using any of these materials. Since the porous insulating layer has low permittivity as compared to a dense insulating layer, capacitance due to an electrode or wiring can be further reduced. Alternatively, the insulating layer 136, the insulating layer 138, and the insulating layer 140 can be formed using an organic insulating material such as polyimide or acrylic. In this embodiment, the case of using silicon oxynitride for the insulating layer 136, silicon nitride oxide for the insulating layer 138, and silicon oxide for the insulating layer 140 will be described. A layered structure of the insulating layer 136, the insulating layer 138, and the insulating layer 140 is employed here; however, an embodiment of the disclosed invention is not limited to this. A single-layer structure, a layered structure of two layers, or a layered structure of four or more layers may be used.

Next, the insulating layer 138 and the insulating layer 140 are subjected to chemical mechanical polishing (CMP) treatment or etching treatment, so that the insulating layer 138 and the insulating layer 140 are flattened (see FIG. 8C). Here, CMP treatment is performed until the insulating layer 138 is partly exposed. When silicon nitride oxide is used for the insulating layer 138 and silicon oxide is used for the insulating layer 140, the insulating layer 138 functions as an etching stopper.

Next, the insulating layer 138 and the insulating layer 140 are subjected to CMP treatment or etching treatment, so that upper surfaces of the gate electrode 128 a and the conductive layer 128 b are exposed (see FIG. 8D). Here, etching treatment is performed until the gate electrode 128 a and the conductive layer 128 b are partly exposed. For the etching treatment, dry etching is preferably performed, but wet etching may be performed. In the step of partly exposing the gate electrode 128 a and the conductive layer 128 b, in order to improve the characteristics of the transistor 162 which is formed later, the surfaces of the insulating layer 136, the insulating layer 138, and the insulating layer 140 are preferably flattened as much as possible.

Through the above steps, the transistor 160 in the lower portion can be formed (see FIG. 8D).

Note that before or after the above steps, a step for forming an additional electrode, wiring, semiconductor layer, or insulating layer may be performed. For example, by employing a multilayer wiring structure in which an insulating layer and a conductive layer are stacked as a wiring structure, a highly integrated semiconductor device can be provided.

<Method for Manufacturing Transistor in Upper Portion>

Next, a method for manufacturing the transistor 162 in the upper portion will be described with reference to FIGS. 9A to 9D and FIGS. 10A to 10C.

First, an oxide semiconductor layer is formed over the gate electrode 128 a, the conductive layer 128 b, the insulating layer 136, the insulating layer 138, the insulating layer 140, and the like and is processed, so that the oxide semiconductor layer 144 is formed (see FIG. 9A). Note that an insulating layer functioning as a base may be formed over the insulating layer 136, the insulating layer 138, and the insulating layer 140 before the oxide semiconductor layer is formed. The insulating layer can be formed by a PVD method such as a sputtering method, a CVD method such as a plasma CVD method, or the like.

An oxide semiconductor to be used preferably contains at least indium (In) or zinc (Zn). In particular, both In and Zn are preferably contained. As a stabilizer for reducing variation in electric characteristics of a transistor including the oxide semiconductor, gallium (Ga) is preferably additionally contained. Tin (Sn) is preferably contained as a stabilizer. Hafnium (Hf) is preferably contained as a stabilizer. Aluminum (Al) is preferably contained as a stabilizer.

As another stabilizer, one or plural kinds of lanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tin), ytterbium (Yb), and lutetium (Lu) may be contained.

As the oxide semiconductor, for example, indium oxide, tin oxide, zinc oxide, a two-component metal oxide such as an In—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, or an In—Ga-based oxide, a three-component metal oxide such as an In—Ga—Zn-based oxide (also referred to as IGZO), an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-based oxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide, or a four-component metal oxide such as an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide can be used.

Note that here, for example, an “In—Ga—Zn-based oxide” means an oxide containing In, Ga, and Zn as its main components and there is no particular limitation on the ratio of In:Ga:Zn. Further, a metal element in addition to In, Ga, and Zn may be contained.

As a material used for the oxide semiconductor layer, a four-component metal oxide material such as an In—Sn—Ga—Zn—O-based material; a three-component metal oxide material such as an In—Ga—Zn—O-based material, an In—Sn—Zn—O-based material, an In—Al—Zn—O-based material, a Sn—Ga—Zn—O-based material, an Al—Ga—Zn—O-based material, or a Sn—Al—Zn—O-based material; a two-component metal oxide material such as an In—Zn—O-based material, a Sn—Zn—O-based material, an Al—Zn—O-based material, a Zn—Mg—O-based material, a Sn—Mg—O-based material, an In—Mg—O-based material, or an In—Ga—O-based material; or a single-component metal oxide material such as an In—O-based material, a Sn—O-based material, or a Zn—O-based material can be used. In addition, the above materials may contain SiO₂. Here, for example, an In—Ga—Zn—O-based material means an oxide film containing indium (In), gallium (Ga), and zinc (Zn), and there is no particular limitation on the composition ratio thereof. Further, the In—Ga—Zn—O-based material may contain an element other than In, Ga, and Zn.

For example, an In—Ga—Zn-based oxide with an atomic ratio of In:Ga:Zn=1:1:1 (=1/3:1/3:1/3) or In:Ga:Zn=2:2:1 (=2/5:2/5:1/5), or any of oxides whose composition is in the neighborhood of the above compositions can be used. Alternatively, an In—Sn—Zn-based oxide with an atomic ratio of In:Sn:Zn=1:1:1 (=1/3:1/3:1/3), In:Sn:Zn=2:1:3 (=1/3:1/6:1/2), or In:Sn:Zn=2:1:5 (=1/4:1/8:5/8), or any of oxides whose composition is in the neighborhood of the above compositions may be used.

However, without limitation to the materials given above, a material with an appropriate composition may be used depending on needed semiconductor characteristics (e.g., mobility, a threshold voltage, and variation). In order to obtain the needed semiconductor characteristics, it is preferable that the carrier concentration, the impurity concentration, the defect density, the atomic ratio between a metal element and oxygen, the interatomic distance, the density, and the like be set to appropriate values.

For example, high mobility can be obtained relatively easily in the case of using an In—Sn—Zn-based oxide. However, also in the case of using an In—Ga—Zn-based oxide, mobility can be increased by reducing the defect density in a bulk.

Note that for example, the expression “the composition of an oxide including In, Ga, and Zn at the atomic ratio of In:Ga:Zn=a:b:c (a+b+c=1) is in the neighborhood of the composition of an oxide including In, Ga, and Zn at the atomic ratio of In:Ga:Zn=A:B:C (A+B+C=1)” means that a, b, and c satisfy the following relation: (a−A)²+(b−B)²+(c−C)²≦r², and r may be 0.05, for example. The same applies to other oxides.

The oxide semiconductor may be either single crystal or non-single-crystal. In the latter case, the oxide semiconductor may be either amorphous or polycrystalline. Further, the oxide semiconductor may have either an amorphous structure including a portion having crystallinity or a non-amorphous structure.

In the case of an oxide semiconductor in an amorphous state, a flat surface can be obtained relatively easily, so that when a transistor is manufactured with the use of such an oxide semiconductor, interface scattering can be reduced, and relatively high mobility can be obtained relatively easily.

In the case of an oxide semiconductor having crystallinity, defects in a bulk can be further reduced, and when a surface flatness is improved, mobility of higher than that of an oxide semiconductor in an amorphous state can be obtained. In order to improve the surface flatness, the oxide semiconductor is preferably formed over a flat surface. Specifically, the oxide semiconductor may be formed over a surface with an average surface roughness (Ra) of less than or equal to 1 nm, preferably less than or equal to 0.3 nm, more preferably less than or equal to 0.1 nm.

Note that “average surface roughness” (Ra) in this specification is obtained by three-dimensionally expanding a centerline average roughness defined by JIS B0601 so as to be applied to a plane. The Ra can be expressed as an “average value of absolute values of deviations from a reference plane to a designated plane”, and is defined by the following Formula 1:

$\begin{matrix} {{Ra} = {\frac{1}{S_{0}}{\int_{x_{2}}^{x_{1}\;}{\int_{y_{2}}^{y_{1}}{{{{f\left( {x,y} \right)} - Z_{0}}}{\mathbb{d}x}{\mathbb{d}y}}}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack \end{matrix}$

In Formula 1, S₀ represents an area of a plane to be measured (a rectangular region which is defined by four points represented by coordinates (x₁, y₁), (x₁, y₂), (x₂, y₁), and (x₂, y₂)), and Z₀ represents an average height of the plane to be measured. Ra can be measured using an atomic force microscope (AFM).

As the oxide semiconductor layer, a thin film including a material expressed as the chemical formula, InMO₃(ZnO)_(m) (m>0), can be used. Here, M represents one or more metal elements selected from Ga, Al, Mn, and Co. For example, M may be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like.

The thickness of the oxide semiconductor layer is preferably greater than or equal to 3 nm and less than or equal to 30 nm. This is because the transistor might be normally on when the oxide semiconductor layer is too thick (e.g., when the thickness is greater than or equal to 50 nm).

The oxide semiconductor layer is preferably formed by a method in which impurities such as hydrogen, water, a hydroxyl group, or hydride do not enter the oxide semiconductor layer. For example, a sputtering method or the like can be used.

In the case where an In—Zn—O-based material is used as an oxide semiconductor, a target therefor has a composition ratio of In:Zn=50:1 to 1:2 in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio), preferably In:Zn=20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2 in a molar ratio), more preferably In:Zn=15:1 to 1.5:1 in an atomic ratio (In₂O₃:ZnO=15:2 to 3:4 in a molar ratio). For example, in a target used for formation of an In—Zn—O-based oxide semiconductor which has an atomic ratio of In:Zn:O=X:Y:Z, the relation of Z>1.5X+Y is satisfied.

Further, an In—Sn—Zn-based oxide can be referred to as ITZO. An oxide target which has an atomic ratio of In, Sn, and Zn of 1:2:2, 2:1:3, 1:1:1, 20:45:35, or the like is used.

In this embodiment, the oxide semiconductor layer is formed by a sputtering method using an In—Ga—Zn—O-based oxide target.

As the In—Ga—Zn—O-based oxide target, for example, an oxide target having a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio] can be used. Note that it is not necessary to limit the material and the composition ratio of the target to the above. For example, an oxide target having a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio] can be used.

The filling rate of the oxide target is greater than or equal to 90% and less than or equal to 100%, preferably greater than or equal to 95% and less than or equal to 99.9%. With the use of the metal oxide target with a high filling rate, a dense oxide semiconductor layer can be formed.

A film formation atmosphere may be a rare gas (typically argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere containing a rare gas and oxygen. An atmosphere of a high-purity gas from which impurities such as hydrogen, water, a hydroxyl group, or hydride are sufficiently removed is preferable, in order to prevent hydrogen, water, a hydroxyl group, hydride, or the like from entering the oxide semiconductor layer.

For example, the oxide semiconductor layer can be formed as follows.

First, the substrate is held in a film formation chamber which is kept under reduced pressure and then is heated so that the substrate temperature reaches a temperature of greater than 200° C. and less than or equal to 500° C., preferably greater than 300° C. and less than or equal to 500° C., more preferably greater than or equal to 350° C. and less than or equal to 450° C.

Then, a high-purity gas from which impurities such as hydrogen, water, a hydroxyl group, or hydride are sufficiently removed is introduced into the film formation chamber from which remaining moisture is being removed, and the oxide semiconductor layer is formed over the substrate with the use of the target. To remove moisture remaining in the film formation chamber, an entrapment vacuum pump such as a cryopump, an ion pump, or a titanium sublimation pump is desirably used as an evacuation unit. Further, an evacuation unit may be a turbo pump provided with a cold trap. In the film formation chamber which is evacuated with a cryopump, for example, impurities such as hydrogen, water, a hydroxyl group, or hydride (preferably, also a compound containing a carbon atom) are removed, whereby the concentration of impurities such as hydrogen, water, a hydroxyl group, or hydride in the oxide semiconductor layer formed in the film formation chamber can be reduced.

In the case where the substrate temperature is low (for example, less than or equal to 100° C.) during film formation, a substance containing a hydrogen atom may enter the oxide semiconductor; thus, it is preferable that the substrate be heated at a temperature in the above range. When the oxide semiconductor layer is formed with the substrate heated at the temperature described above, since the substrate temperature is high, hydrogen bonds are cut by heat and a substance containing a hydrogen atom is less likely to be taken into the oxide semiconductor layer. Therefore, when the oxide semiconductor layer is formed with the substrate heated at the temperature described above, the concentration of impurities such as hydrogen, water, a hydroxyl group, or hydride in the oxide semiconductor layer can be sufficiently reduced. Moreover, damage due to sputtering can be reduced.

As an example of film formation conditions, the following conditions are employed: the distance between the substrate and the target is 60 mm; the pressure is 0.4 Pa; the direct-current (DC) power is 0.5 kW; the substrate temperature is 400° C.; and the film formation atmosphere is an oxygen atmosphere (the proportion of the oxygen flow rate is 100%). Note that a pulse direct current power source is preferable because powder substances (also referred to as particles or dust) generated in film formation can be reduced and the film thickness can be uniform.

Note that before the oxide semiconductor layer is formed by a sputtering method, powdery substances (also referred to as particles or dust) attached on a formation surface of the oxide semiconductor layer are preferably removed by reverse sputtering in which an argon gas is introduced and plasma is generated. The reverse sputtering refers to a method in which voltage is applied to a substrate to generate plasma in the vicinity of the substrate to modify a surface on the substrate side. Note that instead of argon, a gas of nitrogen, helium, oxygen, or the like may be used.

The oxide semiconductor layer can be processed through the steps of forming a mask having a desired shape over the oxide semiconductor layer and etching the oxide semiconductor layer. The mask may be formed by a method such as photolithography. Alternatively, a method such as an inkjet method may be used to form the mask. For the etching of the oxide semiconductor layer, either wet etching or dry etching may be employed. It is needless to say that both of them may be employed in combination.

After that, heat treatment (first heat treatment) may be performed on the oxide semiconductor layer 144. By performing the heat treatment, a substance containing a hydrogen atom in the oxide semiconductor layer 144 can be further removed. The heat treatment is performed in an inert gas atmosphere at greater than or equal to 250° C. and less than or equal to 700° C., preferably greater than or equal to 450° C. and less than or equal to 600° C. or less than a strain point of the substrate. The inert gas atmosphere is preferably an atmosphere which contains nitrogen or a rare gas (e.g., helium, neon, or argon) as its main component and does not contain water, hydrogen, or the like. For example, the purity of nitrogen or a rare gas such as helium, neon, or argon introduced into a heat treatment apparatus is greater than or equal to 6 N (99.9999%), preferably greater than or equal to 7 N (99.99999%) (that is, the concentration of the impurities is less than or equal to 1 ppm, preferably less than or equal to 0.1 ppm).

The heat treatment can be performed in such a way that, for example, an object to be processed is introduced into an electric furnace in which a resistance heating element or the like is used and heated in a nitrogen atmosphere at 450° C. for one hour. The oxide semiconductor layer 144 is not exposed to the air during the heat treatment so that entry of water and hydrogen can be prevented.

The above heat treatment has an effect of removing hydrogen, water, and the like and can be referred to as dehydration treatment, dehydrogenation treatment, or the like. The heat treatment can be performed, for example, before the oxide semiconductor layer is processed into an island shape or after the gate insulating layer is formed. Such dehydration treatment or dehydrogenation treatment may be performed once or plural times.

Next, a conductive layer for forming a source electrode and a drain electrode (including a wiring formed in the same layer as the source electrode and the drain electrode) is formed over the oxide semiconductor layer 144 and the like and is processed, so that the source electrode 142 a and the drain electrode 142 b are formed (see FIG. 9B).

The conductive layer can be formed by a PVD method or a CVD method. As a material for the conductive layer, an element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten; an alloy containing any of these elements as its component; or the like can be used. Further, one or more materials selected from manganese, magnesium, zirconium, beryllium, neodymium, and scandium may be used.

The conductive layer can have a single-layer structure or a layered structure including two or more layers. For example, the conductive layer can have a single-layer structure of a titanium film or a titanium nitride film, a single-layer structure of an aluminum film containing silicon, a two-layer structure in which a titanium film is stacked over an aluminum film, a two-layer structure in which a titanium film is stacked over a titanium nitride film, or a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked. Note that the conductive layer having a single-layer structure of a titanium film or a titanium nitride film has an advantage in that it can be easily processed into the source electrode 142 a and the drain electrode 142 b each having a tapered shape.

Alternatively, the conductive layer may be formed using conductive metal oxide. As the conductive metal oxide, indium oxide (In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), an alloy of indium oxide and tin oxide (In₂O₃—SaO₂, which is abbreviated to ITO in some cases), an alloy of indium oxide and zinc oxide (In₂O₃—ZnO), or any of these metal oxide materials in which silicon or silicon oxide is contained can be used.

The conductive layer is preferably etched so that end portions of the source electrode 142 a and the drain electrode 142 b that are formed are tapered. Here, a taper angle is, for example, preferably greater than or equal to 30° and less than or equal to 60°. The etching is performed so that the end portions of the source electrode 142 a and the drain electrode 142 b are tapered, whereby coverage with the gate insulating layer 146 formed later can be improved and disconnection can be prevented.

The channel length (L) of the transistor in the upper portion is determined by a distance between lower end portions of the source electrode 142 a and the drain electrode 142 b. Note that for light exposure for forming a mask used in the case where a transistor with a channel length (L) of less than 25 nm is formed, it is preferable to use extreme ultraviolet light whose wavelength is as short as several nanometers to several tens of nanometers. In the light exposure by extreme ultraviolet light, the resolution is high and the focus depth is large. For these reasons, the channel length (L) of the transistor to be formed later can be in the range of greater than or equal to 10 nm and less than or equal to 1000 nm (1 μm), and the circuit can operate at higher speed. Moreover, miniaturization can lead to lower power consumption of the semiconductor device.

As an example which is different from that in FIG. 9B, oxide conductive layers can be provided as a source region and a drain region, between the oxide semiconductor layer 144 and the source and drain electrodes. A material of the oxide conductive layer preferably contains zinc oxide as its component and preferably does not contain indium oxide. For such an oxide conductive layer, zinc oxide, zinc aluminum oxide, zinc aluminum oxynitride, zinc gallium oxide, or the like can be used.

For example, the oxide conductive layers which serve as a source region and a drain region, the source electrode 142 a, and the drain electrode 142 b can be formed by forming an oxide conductive film over the oxide semiconductor layer 144, forming a conductive layer over the oxide conductive film, and processing the oxide conductive film and the conductive layer in one photolithography step.

Alternatively, a stacked layer of an oxide semiconductor film and an oxide conductive film is formed and the stacked layer is processed in one photolithography step, so that the island-shaped oxide semiconductor layer 144 and oxide conductive film may be formed. After the source electrode 142 a and the drain electrode 142 b are formed, the island-shaped oxide conductive film is etched using the source electrode 142 a and the drain electrode 142 b as masks, so that the oxide conductive layers which serve as a source region and a drain region can be formed.

Note that when etching treatment for processing the oxide conductive layer is performed, etching conditions (e.g., type of an etching agent, the concentration of an etching agent, and etching time) are adjusted as appropriate in order to prevent excessive etching of the oxide semiconductor layer.

When oxide conductive layers are provided between the oxide semiconductor layer and the source and drain electrodes, a source region and a drain region can have lower resistance and the transistor can operate at high speed. With the structure including the oxide semiconductor layer 144, the oxide conductive layer, and the drain electrode formed using a metal material, withstand voltage of the transistor can be further increased.

It is effective to use the oxide conductive layers for the source region and the drain region in order to improve frequency characteristics of a peripheral circuit (a driver circuit). The contact resistance can be reduced when a metal electrode (e.g., molybdenum or tungsten) and the oxide conductive layer are in contact with each other, as compared to the case where a metal electrode (e.g., molybdenum or tungsten) and the oxide semiconductor layer are in contact with each other. The contact resistance can be reduced by interposing the oxide conductive layers between the oxide semiconductor layer and the source and drain electrodes; accordingly, frequency characteristics of a peripheral circuit (a driver circuit) can be improved.

Next, the gate insulating layer 146 is formed so as to cover the source electrode 142 a and the drain electrode 142 b and to be in contact with part of the oxide semiconductor layer 144 (see FIG. 9C).

The gate insulating layer 146 can be formed by a CVD method, a sputtering method, or the like. The gate insulating layer 146 is preferably formed to contain silicon oxide, silicon nitride, silicon oxynitride, gallium oxide, aluminum oxide, tantalum oxide, hafnium oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y)N_(z)), (x>0, y>0)), hafnium silicate to which nitrogen is added (HfSi_(x)O_(y)N_(z)(x>0, y>0, z>0)), hafnium aluminate to which nitrogen is added (HfAl_(x)O_(y)N_(Z)(x>0, y>0, z>0)), or the like. The gate insulating layer 146 may have a single-layer structure or a layered structure in which these materials are combined. There is no particular limitation on the thickness; however, in the case where the semiconductor device is miniaturized, the thickness is preferably small for ensuring an operation of the transistor. For example, in the case where silicon oxide is used, the thickness can be set to greater than or equal to 1 nm and less than or equal to 100 nm, preferably greater than or equal to 10 nm and less than or equal to 50 nm.

When the gate insulating layer is thin as in the above description, a problem of gate leakage current due to a tunneling effect or the like is caused. In order to solve the problem of gate leakage current, a high permittivity (high-k) material such as hafnium oxide, tantalum oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate to which nitrogen is added (HfSi_(x)O_(y)N_(z) (x>0, y>0, z>0)), or hafnium aluminate to which nitrogen is added (HfAl_(x)O_(y)N_(z) (x>0, y>0, z>0)) is preferably used for the gate insulating layer 146. By using a high-k material for the gate insulating layer 146, electric characteristics can be ensured and the thickness can be large to prevent gate leakage current. Note that a layered structure of a film containing a high-k material and a film containing any one of silicon oxide, silicon nitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, and the like may be employed.

Further, the insulating layer in contact with the oxide semiconductor layer 144 (in this embodiment, the gate insulating layer 146) may be formed using an insulating material containing a Group 13 element and oxygen. Many oxide semiconductor materials contain a Group 13 element, and thus, an insulating material containing a Group 13 element works well with the oxide semiconductor. By using such an insulating material for the insulating layer in contact with the oxide semiconductor, an interface with the oxide semiconductor can be kept in a favorable state.

An insulating material containing a Group 13 element means an insulating material containing one or more Group 13 elements. As an insulating material containing a Group 13 element, gallium oxide, aluminum oxide, aluminum gallium oxide, gallium aluminum oxide, and the like are given. Here, aluminum gallium oxide is a material in which the amount of aluminum is larger than that of gallium in atomic percent, and gallium aluminum oxide is a material in which the amount of gallium is larger than that of aluminum in atomic percent.

For example, in the case of forming a gate insulating layer in contact with an oxide semiconductor layer containing gallium, a material containing gallium oxide may be used as the gate insulating layer, so that favorable characteristics can be maintained at the interface between the oxide semiconductor layer and the gate insulating layer. When the oxide semiconductor layer and the insulating layer containing gallium oxide are provided in contact with each other, pileup of hydrogen at the interface between the oxide semiconductor layer and the insulating layer can be reduced. Note that a similar effect can be obtained in the case where an element belonging to the same group as a constituent element of the oxide semiconductor is used for the insulating layer. For example, it is also effective to form an insulating layer with the use of a material containing aluminum oxide. Aluminum oxide has a property of not easily transmitting water. Thus, it is preferable to use the material containing aluminum oxide in terms of preventing entry of water into the oxide semiconductor layer.

The insulating layer in contact with the oxide semiconductor layer 144 preferably contains oxygen in a proportion higher than that in the stoichiometric proportion by heat treatment in an oxygen atmosphere or oxygen doping. “Oxygen doping” refers to addition of oxygen into a bulk. Note that the term “bulk” is used in order to clarify that oxygen is added not only to a surface of a thin film but also to the inside of the thin film. In addition, “oxygen doping” includes “oxygen plasma doping” in which oxygen which is made to be plasma is added to a bulk. The oxygen doping may be performed using an ion implantation method or an ion doping method.

For example, in the case where the insulating layer in contact with the oxide semiconductor layer 144 is formed of gallium oxide, the composition of gallium oxide can be set to be Ga₂O_(x) (x=3+α, 0<α<1) by heat treatment in an oxygen atmosphere or oxygen doping. In the case where the insulating layer in contact with the oxide semiconductor layer 144 is formed of aluminum oxide, the composition of aluminum oxide can be set to be Al₂O_(x) (x=3+α, 0<α<1) by heat treatment in an oxygen atmosphere or oxygen doping. In the case where the insulating layer in contact with the oxide semiconductor layer 144 is formed of gallium aluminum oxide (aluminum gallium oxide), the composition of gallium aluminum oxide (aluminum gallium oxide) can be set to be Ga_(x)Al_(2−x)O_(3+α) (0<x<2, 0α<1) by heat treatment in an oxygen atmosphere or oxygen doping.

By oxygen doping or the like, an insulating layer including a region where the proportion of oxygen is higher than that in the stoichiometric proportion can be formed. When the insulating layer including such a region is in contact with the oxide semiconductor layer, oxygen that exists excessively in the insulating layer is supplied to the oxide semiconductor layer, and oxygen deficiency in the oxide semiconductor layer or at an interface between the oxide semiconductor layer and the insulating layer is reduced.

The insulating layer including a region where the proportion of oxygen is higher than that in the stoichiometric proportion may be applied to an insulating layer serving as a base film of the oxide semiconductor layer 144 instead of the gate insulating layer 146, or both the gate insulating layer 146 and the base film.

After the gate insulating layer 146 is formed, second heat treatment is desirably performed in an inert gas atmosphere or an oxygen atmosphere. The temperature of the heat treatment is greater than or equal to 200° C. and less than or equal to 450° C., preferably greater than or equal to 250° C. and less than or equal to 350° C. For example, the heat treatment may be performed at 250° C. for one hour in a nitrogen atmosphere. The second heat treatment can reduce variation in electric characteristics of the transistor. Further, in the case where the gate insulating layer 146 contains oxygen, oxygen is supplied to the oxide semiconductor layer 144 which has been subjected to the dehydration treatment or dehydrogenation treatment to compensate for oxygen deficiency in the oxide semiconductor layer 144, so that an i-type (intrinsic) or substantially i-type oxide semiconductor layer can be formed.

Note that in this embodiment, the second heat treatment is performed after the gate insulating layer 146 is formed; the timing of the second heat treatment is not limited thereto. For example, the second heat treatment may be performed after the gate electrode is formed. Alternatively, the second heat treatment may be performed following the first heat treatment, the first heat treatment may double as the second heat treatment, or the second heat treatment may double as the first heat treatment.

Next, a conductive layer for forming a gate electrode (including a wiring formed using the same layer as the gate electrode) is formed and is processed, so that the gate electrode 148 a and the conductive layer 148 b are formed (see FIG. 9D).

The gate electrode 148 a and the conductive layer 148 b can be formed using a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, neodymium, or scandium, or an alloy material containing any of these materials as its main component. Note that the gate electrode 148 a and the conductive layer 148 b may have a single-layer structure or a layered structure.

Next, the insulating layer 150 is formed over the gate insulating layer 146, the gate electrode 148 a, and the conductive layer 148 b (see FIG. 10A). The insulating layer 150 can be formed by a PVD method, a CVD method, or the like. The insulating layer 150 can be formed using a material including an inorganic insulating material such as silicon oxide, silicon oxynitride, silicon nitride, hafnium oxide, gallium oxide, or aluminum oxide. Note that for the insulating layer 150, a material with a low permittivity may be preferably used or a structure with a low permittivity (e.g., a porous structure) may be preferably employed. This is because by reducing the permittivity of the insulating layer 150, capacitance between wirings and electrodes can be reduced, which will increase operation speed. Note that although the insulating layer 150 has a single-layer structure in this embodiment, an embodiment of the disclosed invention is not limited to this. The insulating layer 150 may have a layered structure including two or more layers.

Next, an opening reaching the source electrode 142 a is formed in the gate insulating layer 146 and the insulating layer 150. Then, the wiring 154 connected to the source electrode 142 a is formed over the insulating layer 150 (see FIG. 10B). The opening is formed by selective etching using a mask or the like.

A conductive layer is formed by a PVD method or a CVD method and then is patterned, so that the wiring 154 is formed. As a material for the conductive layer, an element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten; an alloy containing any of these elements as its component; or the like can be used. Further, one or more materials selected from manganese, magnesium, zirconium, beryllium, neodymium, and scandium may be used.

Specifically, it is possible to employ a method, for example, in which a thin titanium film (about 5 nm) is formed in a region including the opening of the insulating layer 150 by a PVD method, and an aluminum film is formed to fill the opening after the thin titanium film is formed by a PVD method. Here, the titanium film formed by a PVD method has a function of reducing an oxide film (e.g., a native oxide film) formed on a surface where the titanium film is formed, to decrease contact resistance with a lower electrode (here, the source electrode 142 a). In addition, hillock of the aluminum film can be prevented. A copper film may be formed by a plating method after the formation of the barrier film of titanium, titanium nitride, or the like.

The opening formed in the insulating layer 150 is preferably formed in a region overlapping with the conductive layer 128 b. With the opening in such a region, the element area can be prevented from increasing owing to a contact region.

A position where the impurity region 126 and the source electrode 142 a are connected and a position where the source electrode 142 a and the wiring 154 are connected may overlap with each other without using the conductive layer 128 b; such a case will be described. In that case, an opening (also referred to as a contact in a lower portion) is formed in the insulating layer 136, the insulating layer 138, and the insulating layer 140 that are formed over the impurity region 126, and the source electrode 142 a is formed in the contact in the lower portion. After that, an opening (also referred to as a contact in an upper portion) is formed in a region overlapping with the contact in the lower portion in the gate insulating layer 146 and the insulating layer 150, and then the wiring 154 is formed. When the contact in the upper portion is formed in the region overlapping with the contact in the lower portion, the source electrode 142 a formed in the contact in the lower portion might be disconnected by etching. In order to avoid the disconnection, the contact in the lower portion and the contact in the upper portion may be formed so as not to overlap with each other, but a problem of the increase in the element area occurs.

As described in this embodiment, with the use of the conductive layer 128 b, the contact in the upper portion can be formed without disconnection of the source electrode 142 a. Thus, the contact in the lower portion and the contact in the upper portion can be formed overlapping with each other, so that the element area can be prevented from increasing owing to a contact region. In other words, the degree of integration of the semiconductor device can be increased.

Next, the insulating layer 156 is formed so as to cover the wiring 154 (see FIG. 10C).

Through the above process, the capacitor 164 and the transistor 162 including the highly purified oxide semiconductor layer 144 are completed (see FIG. 10C).

Since the oxide semiconductor layer 144 is a highly purified oxide semiconductor layer in the transistor 162 described in this embodiment, the hydrogen concentration thereof is less than or equal to 5×10¹⁹ atoms/cm³, preferably less than or equal to 5×10¹⁸ atoms/cm³, more preferably less than or equal to 5×10¹⁷ atoms/cm³. In addition, the value of the carrier density of the oxide semiconductor layer 144 is sufficiently low (e.g., less than 1×10¹²/cm³, preferably less than 1.45×10¹⁰/cm³) as compared with that of a general silicon wafer (approximately 1×10¹⁴/cm³). Accordingly, the off-state current is also sufficiently small. For example, the off-state current (here, current per micrometer (μm) of channel width) of the transistor 162 at room temperature (25° C.) is less than or equal to 100 zA (1 zA (zeptoampere) is 1×10⁻²¹ A), preferably less than or equal to 10 zA.

With the use of the highly purified intrinsic oxide semiconductor layer 144, the off-state current of the transistor can be sufficiently reduced easily. By using such a transistor, a semiconductor device in which stored data can be held for an extremely long time can be obtained.

In addition, in the semiconductor device described in this embodiment, a wiring can be shared; thus, a semiconductor device with sufficiently increased degree of integration can be achieved.

The structures, methods, and the like described in this embodiment can be combined as appropriate with any of the structures, methods, and the like described in the other embodiments.

Embodiment 3

In this embodiment, the case where the semiconductor device described in any of the above embodiments is applied to electronic devices will be described with reference to FIGS. 11A to 11F. In this embodiment, the semiconductor device described in any of the above embodiments can be applied to electronic device such as computer, a mobile phone set (also referred to as a mobile phone or a mobile phone device), a personal digital assistant (including a portable game machine, an audio reproducing device, and the like), a camera such as a digital camera or a digital video camera, an electronic paper, and a television device (also referred to as a television or a television receiver).

FIG. 11A illustrates a laptop personal computer, which includes a housing 701, a housing 702, a display portion 703, a keyboard 704, and the like. The semiconductor device described in any of the above embodiments is provided in at least one of the housing 701 and the housing 702. Therefore, a laptop personal computer in which writing and reading of data are performed at high speed, data is stored for a long time, and power consumption is sufficiently reduced can be realized.

FIG. 11B illustrates a personal digital assistant (PDA). A main body 711 is provided with a display portion 713, an external interface 715, operation buttons 714, and the like. Further, a stylus 712 and the like for an operation of the personal digital assistant are provided. In the main body 711, the semiconductor device described in any of the above embodiments is provided. Therefore, a personal digital assistant in which writing and reading of data are performed at high speed, data is stored for a long time, and power consumption is sufficiently reduced can be realized.

FIG. 11C illustrates an electronic book reader 720 incorporating an electronic paper, which includes two housings, a housing 721 and a housing 723. The housing 721 and the housing 723 are provided with a display portion 725 and a display portion 727, respectively. The housings 721 and 723 are connected by a hinge portion 737 and can be opened or closed with the hinge portion 737 as an axis. The housing 721 is provided with a power supply 731, an operation key 733, a speaker 735, and the like. At least one of the housings 721 and 723 is provided with the semiconductor device described in any of the above embodiments. Therefore, an electronic book reader in which writing and reading of data are performed at high speed, data is stored for a long time, and power consumption is sufficiently reduced can be realized.

FIG. 11D illustrates a mobile phone set, which includes two housings, a housing 740 and a housing 741. The housings 740 and 741 in a state where they are developed as illustrated in FIG. 11D can be slid so that one is lapped over the other. Therefore, the size of the mobile phone set can be reduced, which makes the mobile phone set suitable for being carried around. The housing 741 includes a display panel 742, a speaker 743, a microphone 744, an operation key 745, a pointing device 746, a camera lens 747, an external connection terminal 748, and the like. The housing 740 includes a solar cell 749 for charging the mobile phone set, an external memory slot 750, and the like. In addition, an antenna is incorporated in the housing 741. At least one of the housings 740 and 741 is provided with the semiconductor device described in any of the above embodiments. Therefore, a mobile phone set in which writing and reading of data are performed at high speed, data is stored for a long time, and power consumption is sufficiently reduced can be realized.

FIG. 11E illustrates a digital camera, which includes a main body 761, a display portion 767, an eyepiece 763, an operation switch 764, a display portion 765, a battery 766, and the like. In the main body 761, the semiconductor device described in any of the above embodiments is provided. Therefore, a digital camera in which writing and reading of data are performed at high speed, data is stored for a long time, and power consumption is sufficiently reduced can be realized.

FIG. 11F illustrates a television set 770, which includes a housing 771, a display portion 773, a stand 775, and the like. The television set 770 can be operated with a switch provided for the housing 771 or a remote controller 780. The semiconductor device described in any of the above embodiments is mounted on the housing 771 and the remote controller 780. Therefore, a television set in which writing and reading of data are performed at high speed, data is stored for a long time, and power consumption is sufficiently reduced can be realized.

As described above, the semiconductor device described in any of the above embodiments is mounted in the electronic devices described in this embodiment. Therefore, electronic devices with low power consumption can be realized.

Embodiment 4

In this embodiment, the transistor including an oxide semiconductor as a semiconductor material, which has been described in Embodiments 1 to 3, will be described in detail. Specifically, as the oxide semiconductor, an oxide including a crystal with c-axis alignment (also referred to as C-Axis Aligned Crystal (CAAC)), which has a triangular or hexagonal atomic arrangement when seen from the direction of an a-b plane, a surface, or an interface will be described. In the crystal, metal atoms are arranged in a layered manner, or metal atoms and oxygen atoms are arranged in a layered manner along the c-axis, and the direction of the a-axis or the b-axis is varied in the a-b plane (the crystal rotates around the c-axis).

In a broad sense, an oxide including CAAC means a non-single-crystal oxide including a phase which has a triangular, hexagonal, regular triangular, or regular hexagonal atomic arrangement when seen from the direction perpendicular to the a-b plane and in which metal atoms are arranged in a layered manner or metal atoms and oxygen atoms are arranged in a layered manner when seen from the direction perpendicular to the c-axis direction.

The CAAC is not a single crystal, but this does not mean that the CAAC is composed of only an amorphous component. Although the CAAC includes a crystallized portion (crystalline portion), a boundary between one crystalline portion and another crystalline portion is not clear in some cases.

In the case where oxygen is included in the CAAC, nitrogen may be substituted for part of oxygen included in the CAAC. The c-axes of individual crystalline portions included in the CAAC may be aligned in one direction (e.g., a direction perpendicular to a surface of a substrate over which the CAAC is formed or a surface of the CAAC). Alternatively, the normals of the a-b planes of the individual crystalline portions included in the CAAC may be aligned in one direction (e.g., a direction perpendicular to a surface of a substrate over which the CAAC is formed or a surface of the CAAC).

The CAAC becomes a conductor, a semiconductor, or an insulator depending on its composition or the like. The CAAC transmits or does not transmit visible light depending on its composition or the like.

As an example of such CAAC, there is a crystal which is formed into a film shape and has a triangular or hexagonal atomic arrangement when observed from the direction perpendicular to a surface of the film or a surface of a supporting substrate, and in which metal atoms are arranged in a layered manner or metal atoms and oxygen atoms (or nitrogen atoms) are arranged in a layered manner when a cross section of the film is observed.

An example of a crystal structure of the CAAC will be described in detail with reference to FIGS. 12A to 12E, FIGS. 13A to 13C, and FIGS. 14A to 14C. In FIGS. 12A to 12E, FIGS. 13A to 13C, and FIGS. 14A to 14C, the vertical direction corresponds to the c-axis direction and a plane perpendicular to the c-axis direction corresponds to the a-b plane, unless otherwise specified. When the expressions “an upper half” and “a lower half” are simply used, they mean an upper half above the a-b plane and a lower half below the a-b plane (an upper half and a lower half with respect to the a-b plane).

FIG. 12A illustrates a structure including one hexacoordinate In atom and six tetracoordinate oxygen (hereinafter referred to as tetracoordinate O) atoms proximate to the In atom. Here, a structure including one metal atom and oxygen atoms proximate thereto is referred to as a small group. The structure in FIG. 12A is actually an octahedral structure, but is illustrated as a planar structure for simplicity. Note that three tetracoordinate O atoms exist in each of an upper half and a lower half in FIG. 12A. In the small group illustrated in FIG. 12A, electric charge is 0.

FIG. 12B illustrates a structure including one pentacoordinate Ga atom, three tricoordinate oxygen (hereinafter referred to as tricoordinate 0) atoms proximate to the Ga atom, and two tetracoordinate O atoms proximate to the Ga atom. All the tricoordinate O atoms exist on the a-b plane. One tetracoordinate O atom exists in each of an upper half and a lower half in FIG. 12B. An In atom can also have the structure illustrated in FIG. 12B because an In atom can have five ligands. In the small group illustrated in FIG. 12B, electric charge is 0.

FIG. 12C illustrates a structure including one tetracoordinate Zn atom and four tetracoordinate O atoms proximate to the Zn atom. In FIG. 12C, one tetracoordinate O atom exists in an upper half and three tetracoordinate O atoms exist in a lower half. Alternatively, three tetracoordinate O atoms may exist in the upper half and one tetracoordinate O atom may exist in the lower half in FIG. 12C. In the small group illustrated in FIG. 12C, electric charge is 0.

FIG. 12D illustrates a structure including one hexacoordinate Sn atom and six tetracoordinate O atoms proximate to the Sn atom. In FIG. 12D, three tetracoordinate O atoms exist in each of an upper half and a lower half. In the small group illustrated in FIG. 12D, electric charge is +1.

FIG. 12E illustrates a small group including two Zn atoms. In FIG. 12E, one tetracoordinate O atom exists in each of an upper half and a lower half. In the small group illustrated in FIG. 12E, electric charge is −1.

Here, a plurality of small groups form a medium group, and a plurality of medium groups form a large group (also referred to as a unit cell).

Now, a rule of bonding between the small groups will be described. The three O atoms in the upper half with respect to the hexacoordinate In atom in FIG. 12A each have three proximate In atoms in the downward direction, and the three O atoms in the lower half each have three proximate In atoms in the upward direction. The one O atom in the upper half with respect to the pentacoordinate Ga atom in FIG. 12B has one proximate Ga atom in the downward direction, and the one O atom in the lower half has one proximate Ga atom in the upward direction. The one O atom in the upper half with respect to the tetracoordinate Zn atom in FIG. 12C has one proximate Zn atom in the downward direction, and the three O atoms in the lower half each have three proximate Zn atoms in the upward direction. In this manner, the number of the tetracoordinate O atoms above the metal atom is equal to the number of the metal atoms proximate to and below each of the tetracoordinate O atoms. Similarly, the number of the tetracoordinate O atoms below the metal atom is equal to the number of the metal atoms proximate to and above each of the tetracoordinate O atoms. Since the coordination number of the tetracoordinate O atom is 4, the sum of the number of the metal atoms proximate to and below the O atom and the number of the metal atoms proximate to and above the O atom is 4. Accordingly, when the sum of the number of tetracoordinate O atoms above a metal atom and the number of tetracoordinate O atoms below another metal atom is 4, the two kinds of small groups including the metal atoms can be bonded. For example, in the case where the hexacoordinate metal (In or Sn) atom is bonded through three tetracoordinate O atoms in the lower half, it is bonded to the pentacoordinate metal (Ga or In) atom or the tetracoordinate metal (Zn) atom.

Metal atoms having the above coordination numbers are bonded to each other through a tetracoordinate O atom in the c-axis direction. In addition to the above, a medium group can be formed in a different manner by combining a plurality of small groups so that the total electric charge of the layered structure is 0.

FIG. 13A illustrates a model of a medium group included in a layered structure of an In—Sn—Zn—O-based material. FIG. 13B illustrates a large group including three medium groups. Note that FIG. 13C illustrates an atomic arrangement in the case where the layered structure in FIG. 13B is observed from the c-axis direction.

In FIG. 13A, a tricoordinate O atom is omitted for simplicity, and a tetracoordinate O atom is illustrated by a circle; the number in the circle shows the number of tetracoordinate O atoms. For example, three tetracoordinate O atoms existing in each of an upper half and a lower half with respect to a Sn atom are denoted by circled 3. Similarly, in FIG. 13A, one tetracoordinate O atom existing in each of an upper half and a lower half with respect to an In atom is denoted by circled 1. FIG. 13A also illustrates a Zn atom proximate to one tetracoordinate O atom in a lower half and three tetracoordinate O atoms in an upper half, and a Zn atom proximate to one tetracoordinate O atom in an upper half and three tetracoordinate O atoms in a lower half.

In the medium group included in the layered structure of the In—Sn—Zn—O-based material in FIG. 13A, in the order starting from the top, a Sn atom proximate to three tetracoordinate O atoms in each of an upper half and a lower half is bonded to an In atom proximate to one tetracoordinate O atom in each of an upper half and a lower half, the In atom is bonded to a Zn atom proximate to three tetracoordinate O atoms in an upper half, the Zn atom is bonded to an In atom proximate to three tetracoordinate O atoms in each of an upper half and a lower half through one tetracoordinate O atom in a lower half with respect to the Zn atom, the In atom is bonded to a small group that includes two Zn atoms and is proximate to one tetracoordinate O atom in an upper half, and the small group is bonded to a Sn atom proximate to three tetracoordinate O atoms in each of an upper half and a lower half through one tetracoordinate O atom in a lower half with respect to the small group. A plurality of such medium groups are bonded, so that a large group is formed.

Here, electric charge for one bond of a tricoordinate O atom and electric charge for one bond of a tetracoordinate O atom can be assumed to be −0.667 and −0.5, respectively. For example, electric charge of a (hexacoordinate or pentacoordinate) In atom, electric charge of a (tetracoordinate) Zn atom, and electric charge of a (pentacoordinate or hexacoordinate) Sn atom are +3, +2, and +4, respectively. Accordingly, electric charge in a small group including a Sn atom is +1. Therefore, electric charge of −1 which cancels +1, is needed to form a layered structure including a Sn atom. As a structure having electric charge of −1, the small group including two Zn atoms as illustrated in FIG. 12E can be given. For example, with one small group including two Zn atoms, electric charge of one small group including a Sn atom can be cancelled, so that the total electric charge of the layered structure can be 0.

When the large group illustrated in FIG. 13B is repeated, an In—Sn—Zn—O-based crystal (In₂SnZn₃O₈) can be obtained. Note that a layered structure of the obtained In—Sn—Zn—O-based crystal can be expressed as a composition formula, In₂SnZn₂O₇(ZnO)_(m) (m is 0 or a natural number).

The above-described rule also applies to the following oxides: a four-component metal oxide such as an In—Sn—Ga—Zn-based oxide; a three-component metal oxide such as an In—Ga—Zn-based oxide (also referred to as IGZO), an In—Al—Zn-based oxide, a Sn—Ga—Zn-based oxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide; a two-component metal oxide such as an In—Zn-based oxide, a Sn—Zn based oxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, or an In—Ga-based oxide; and the like.

As an example, FIG. 14A illustrates a model of a medium group included in a layered structure of an In—Ga—Zn—O-based material.

In the medium group included in the layered structure of the In—Ga—Zn—O-based material in FIG. 14A, in the order starting from the top, an In atom proximate to three tetracoordinate O atoms in each of an upper half and a lower half is bonded to a Zn atom proximate to one tetracoordinate O atom in an upper half, the Zn atom is bonded to a Ga atom proximate to one tetracoordinate O atom in each of an upper half and a lower half through three tetracoordinate O atoms in a lower half with respect to the Zn atom, and the Ga atom is bonded to an In atom proximate to three tetracoordinate O atoms in each of an upper half and a lower half through one tetracoordinate O atom in a lower half with respect to the Ga atom. A plurality of such medium groups are bonded, so that a large group is formed.

FIG. 14B illustrates a large group including three medium groups. Note that FIG. 14C illustrates an atomic arrangement in the case where the layered structure in FIG. 14B is observed from the c-axis direction.

Here, since electric charge of a (hexacoordinate or pentacoordinate) In atom, electric charge of a (tetracoordinate) Zn atom, and electric charge of a (pentacoordinate) Ga atom are +3, +2, and +3, respectively, electric charge of a small group including any of an In atom, a Zn atom, and a Ga atom is 0. As a result, the total electric charge of a medium group having a combination of such small groups is always 0.

In order to form the layered structure of the In—Ga—Zn—O-based material, a large group can be formed using not only the medium group illustrated in FIG. 14A but also a medium group in which the arrangement of the In atom, the Ga atom, and the Zn atom is different from that in FIG. 14A.

Embodiment 5

In this embodiment, mobility of the transistor including an oxide semiconductor for a channel formation region, which has been described in Embodiments 1 to 4, will be described.

The actually measured field-effect mobility of an insulated gate transistor can be lower than its original mobility because of a variety of reasons; this phenomenon occurs not only in the case of using an oxide semiconductor. One of the reasons that reduce the mobility is a defect inside a semiconductor or a defect at an interface between the semiconductor and an insulating film. When a Levinson model is used, the field-effect mobility on the assumption that no defect exists inside the semiconductor can be calculated theoretically.

Assuming that the original mobility and the measured field-effect mobility of a semiconductor are μ₀ and μ, respectively, and a potential barrier (such as a grain boundary) exists in the semiconductor, the measured field-effect mobility can be expressed as the following formula.

$\begin{matrix} {\mu = {\mu_{0}{\exp\left( {- \frac{E}{kT}} \right)}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack \end{matrix}$

Here, E represents the height of the potential barrier, k represents the Boltzmann constant, and T represents the absolute temperature. When the potential barrier is assumed to be attributed to a defect, the height of the potential barrier can be expressed as the following formula according to the Levinson model.

$\begin{matrix} {E = {\frac{e^{2}N^{2}}{8ɛ\; n} = \frac{e^{3}N^{2}t}{8ɛ\; C_{ox}V_{g}}}} & \left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack \end{matrix}$

Here, e represents the elementary electric charge, N represents the average defect density per unit area in a channel formation region, ε represents the permittivity of the semiconductor, n represents the number of carriers per unit area in the channel formation region, C_(ox) represents the capacitance per unit area, V_(g) represents the gate voltage, and t represents the thickness of the channel formation region. In the case where the thickness of the semiconductor layer is less than or equal to 30 nm, the thickness of the channel formation region may be regarded as being the same as the thickness of the semiconductor layer. The drain current I_(d) in a linear region can be expressed as the following formula.

$\begin{matrix} {I_{d} = {\frac{W\;\mu\; V_{g}C_{d}C_{ox}}{L}{\exp\left( {- \frac{E}{kT}} \right)}}} & \left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack \end{matrix}$

Here, L represents the channel length and W represents the channel width, and L and W are each 10 μm. In addition, V_(d) represents the drain voltage. When dividing both sides of the above equation by V_(g) and then taking logarithms of both sides, the following formula can be obtained.

$\begin{matrix} \begin{matrix} {{\ln\left( \frac{I_{d}}{V_{g}} \right)} = {{\ln\left( \frac{W\;\mu\; V_{d}C_{ox}}{L} \right)} - \frac{E}{kT}}} \\ {= {{\ln\left( \frac{W\;\mu\; V_{d}C_{ox}}{L} \right)} - \frac{e^{3}N^{2}t}{8{kT}_{ɛ}C_{ox}V_{g}}}} \end{matrix} & \left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack \end{matrix}$

The right side of Formula 5 is a function of V_(g). From the formula, it is found that the defect density N can be obtained from the slope of a line in a graph which is obtained by plotting actual measured values with In(I_(d)/V_(g)) as the ordinate and 1/V_(g) as the abscissa. That is, the defect density can be evaluated from the I_(d)−V_(g) characteristics of the transistor. The defect density N of an oxide semiconductor in which the ratio of indium (In), tin (Sn), and zinc (Zn) is 1:1:1 is approximately 1×10¹²/cm².

On the basis of the defect density obtained in this manner, or the like, μ₀ can be calculated to be 120 cm²/Vs from Formula 2 and Formula 3. The measured mobility of an In—Sn—Zn oxide including a defect is approximately 40 cm²/Vs. However, assuming that no defect exists inside the semiconductor and at the interface between the semiconductor and an insulating film, the mobility μ₀ of the oxide semiconductor is expected to be 120 cm²/Vs.

Note that even when no defect exists inside a semiconductor, scattering at an interface between a channel formation region and a gate insulating layer affects the transport property of the transistor. In other words, the mobility μ₁ at a position that is distance x away from the interface between the channel formation region and the gate insulating layer can be expressed as the following formula.

$\begin{matrix} {\frac{1}{\mu_{1}} = {\frac{1}{\mu_{0}} + {\frac{D}{B}{\exp\left( {- \frac{x}{l}} \right)}}}} & \left\lbrack {{Formula}\mspace{14mu} 6} \right\rbrack \end{matrix}$

Here, D represents the electric field in the gate direction, and B and l are constants. B and l can be obtained from actual measurement results; according to the above measurement results, B is 4.75×10⁷ cm/s and l is 10 nm (the depth to which the influence of interface scattering reaches). When D is increased (i.e., when the gate voltage is increased), the second term of Formula 6 is increased and accordingly the mobility μ₁ is decreased.

Calculation results of the mobility μ₂ of a transistor whose channel formation region includes an ideal oxide semiconductor without a defect inside the semiconductor are shown in FIG. 15. For the calculation, device simulation software Sentaurus Device manufactured by Synopsys, Inc. was used, and the band gap, the electron affinity, the relative permittivity, and the thickness of the oxide semiconductor were assumed to be 2.8 eV, 4.7 eV, 15, and 15 nm, respectively. These values were obtained by measurement of a thin film that was formed by a sputtering method.

Further, the work functions of a gate electrode, a source electrode, and a drain electrode were assumed to be 5.5 eV, 4.6 eV, and 4.6 eV, respectively. The thickness of a gate insulating layer was assumed to be 100 nm, and the relative permittivity thereof was assumed to be 4.1. The channel length and the channel width were each assumed to be 10 μm, and the drain voltage V_(d) was assumed to be 0.1 V.

As shown in FIG. 15, a peak of the mobility is more than or equal to 100 cm²/Vs at a gate voltage that is a little over 1 V and the mobility is decreased as the gate voltage becomes higher because the influence of interface scattering is increased. Note that in order to reduce interface scattering, it is desirable that a surface of the semiconductor layer be flat at the atomic level (atomic layer flatness).

Calculation results of characteristics of minute transistors which are manufactured using an oxide semiconductor having such a mobility are shown in FIGS. 16A to 16C, FIGS. 17A to 17C, and FIGS. 18A to 18C. FIGS. 19A and 19B illustrate cross-sectional structures of the transistors used for the calculation. The transistors illustrated in FIGS. 19A and 19B each include a semiconductor region 103 a and a semiconductor region 103 c which have n⁺-type conductivity in an oxide semiconductor layer. The resistivities of the semiconductor region 103 a and the semiconductor region 103 c are 2×10⁻³ Ωcm.

The transistor illustrated in FIG. 19A is formed over a base insulating film 101 and an embedded insulator 102 which is embedded in the base insulating film 101 and formed of aluminum oxide. The transistor includes the semiconductor region 103 a, the semiconductor region 103 c, an intrinsic semiconductor region 103 b serving as a channel formation region therebetween, and a gate electrode 105. The width of the gate electrode 105 is 33 nm.

A gate insulating layer 104 is formed between the gate electrode 105 and the semiconductor region 103 b. In addition, a sidewall insulator 106 a and a sidewall insulator 106 b are formed on both side surfaces of the gate electrode 105, and an insulator 107 is formed over the gate electrode 105 so as to prevent a short circuit between the gate electrode 105 and another wiring. The sidewall insulator has a width of 5 nm A source electrode 108 a and a drain electrode 108 b are provided in contact with the semiconductor region 103 a and the semiconductor region 103 c, respectively. Note that the channel width of this transistor is 40 nm.

The transistor illustrated in FIG. 19B is the same as the transistor illustrated in FIG. 19A in that it is formed over the base insulating film 101 and the embedded insulator 102 formed of aluminum oxide and that it includes the semiconductor region 103 a, the semiconductor region 103 c, the intrinsic semiconductor region 103 b provided therebetween, the gate electrode 105 having a width of 33 nm, the gate insulating layer 104, the sidewall insulator 106 a, the sidewall insulator 106 b, the insulator 107, the source electrode 108 a, and the drain electrode 108 b.

The transistor illustrated in FIG. 19A is different from the transistor illustrated in FIG. 19B in the conductivity type of semiconductor regions under the sidewall insulator 106 a and the sidewall insulator 106 b. In the transistor illustrated in FIG. 19A, the semiconductor regions under the sidewall insulator 106 a and the sidewall insulator 106 b are part of the semiconductor region 103 a having n⁺-type conductivity and part of the semiconductor region 103 c having n⁺-type conductivity, whereas in the transistor illustrated in FIG. 19B, the semiconductor regions under the sidewall insulator 106 a and the sidewall insulator 106 b are part of the intrinsic semiconductor region 103 b. In other words, in the semiconductor layer of FIG. 19B, a region having a width of L_(off) which overlaps with neither the semiconductor region 103 a (the semiconductor region 103 c) nor the gate electrode 105 is provided. This region is called an offset region, and the width L_(off) is called an offset length. As is seen from the drawing, the offset length is equal to the width of the sidewall insulator 106 a (the sidewall insulator 106 b).

The other parameters used in calculation are as described above. For the calculation, device simulation software Sentaurus Device manufactured by Synopsys, Inc. was used. FIGS. 16A to 16C show the gate voltage (V_(g): a potential difference between the gate electrode and the source) dependence of the drain current (I_(d), a solid line) and the mobility (μ, a dotted line) of the transistor having the structure illustrated in FIG. 19A. The drain current I_(d) is obtained by calculation under the assumption that the drain voltage (a potential difference between the drain and the source) is +1 V and the mobility μ is obtained by calculation under the assumption that the drain voltage is +0.1 V.

FIG. 16A shows the gate voltage dependence of the transistor in the case where the thickness of the gate insulating layer is 15 nm, FIG. 16B shows that of the transistor in the case where the thickness of the gate insulating layer is 10 nm, and FIG. 16C shows that of the transistor in the case where the thickness of the gate insulating layer is 5 nm. As the gate insulating layer is thinner, the drain current I_(d) (off-state current) particularly in an off state is significantly decreased. In contrast, there is no noticeable change in the peak value of the mobility μ and the drain current I_(d) in an on state (on-state current). The graphs show that the drain current exceeds 10 μA, which is required in a transistor used in a memory cell and the like, at a gate voltage of around 1 V.

FIGS. 17A to 17C show the gate voltage V_(g) dependence of the drain current I_(d) (a solid line) and the mobility μ (a dotted line) of the transistor having the structure illustrated in FIG. 19B where the offset length L_(off) is 5 nm. The drain current I_(d) is obtained by calculation under the assumption that the drain voltage is +1 V and the mobility μ is obtained by calculation under the assumption that the drain voltage is +0.1 V. FIG. 17A shows the gate voltage dependence of the transistor in the case where the thickness of the gate insulating layer is 15 nm, FIG. 17B shows that of the transistor in the case where the thickness of the gate insulating layer is 10 nm, and FIG. 17C shows that of the transistor in the case where the thickness of the gate insulating layer is 5 nm.

Further, FIGS. 18A to 18C show the gate voltage dependence of the drain current I_(d) (a solid line) and the mobility μ (a dotted line) of the transistor having the structure illustrated in FIG. 19B where the offset length L_(off) is 15 nm. The drain current I_(d) is obtained by calculation under the assumption that the drain voltage is +1 V and the mobility μ is obtained by calculation under the assumption that the drain voltage is +0.1 V. FIG. 18A shows the gate voltage dependence of the transistor in the case where the thickness of the gate insulating layer is 15 nm, FIG. 18B shows that of the transistor in the case where the thickness of the gate insulating layer is 10 nm, and FIG. 18C shows that of the transistor in the case where the thickness of the gate insulating layer is 5 nm.

In either of the structures, as the gate insulating layer is thinner, the off-state current is significantly decreased, whereas no noticeable change arises in the peak value of the mobility μ and the on-state current.

Note that the peak of the mobility μ is approximately 80 cm²/Vs in FIGS. 16A to 16C, approximately 60 cm²/Vs in FIGS. 17A to 17C, and approximately 40 cm²/Vs in FIGS. 18A to 18C; thus, the peak of the mobility μ is decreased as the offset length L_(off) is increased. Further, the same applies to the off-state current. The on-state current is also decreased as the offset length L_(off) is increased; however, the decrease in the on-state current is much more gradual than the decrease in the off-state current. Further, the graphs show that in either of the structures, the drain current exceeds 10 μA, which is required in a transistor used in a memory cell and the like, at a gate voltage of around 1 V.

Embodiment 6

The transistor in which an oxide semiconductor containing In, Sn, and Zn as main components is used as a channel formation region and which has been described in Embodiments 1 to 5 can have favorable characteristics by depositing the oxide semiconductor while heating a substrate or by performing heat treatment after an oxide semiconductor film is formed. Note that a main component refers to an element contained in a composition at greater than or equal to 5 at. %.

By intentionally heating the substrate after formation of the oxide semiconductor film containing In, Sn, and Zn as main components, the field-effect mobility of the transistor can be improved. Further, the threshold voltage of the transistor can be positively shifted to make the transistor normally off.

As an example, FIGS. 20A to 20C each show characteristics of a transistor in which an oxide semiconductor film containing In, Sn, and Zn as main components and having a channel length L of 3 μm and a channel width W of 10 μm, and a gate insulating layer with a thickness of 100 nm are used. Note that V_(d) was set to 10 V.

FIG. 20A shows characteristics of a transistor whose oxide semiconductor film containing In, Sn, and Zn as main components was formed by a sputtering method without heating a substrate intentionally. The field-effect mobility of the transistor is 18.8 cm²/Vsec. On the other hand, when the oxide semiconductor film containing In, Sn, and Zn as main components is formed while heating the substrate intentionally, the field-effect mobility can be improved. FIG. 20B shows characteristics of a transistor whose oxide semiconductor film containing In, Sn, and Zn as main components was formed while heating a substrate at 200° C. The field-effect mobility of the transistor is 32.2 cm²/Vsec.

The field-effect mobility can be further improved by performing heat treatment after formation of the oxide semiconductor film containing In, Sn, and Zn as main components. FIG. 20C shows characteristics of a transistor whose oxide semiconductor film containing In, Sn, and Zn as main components was formed by sputtering at 200° C. and then subjected to heat treatment at 650° C. The field-effect mobility of the transistor is 34.5 cm²/Vsec.

The intentional heating of the substrate is expected to have an effect of reducing moisture taken into the oxide semiconductor film during the formation by sputtering. Further, the heat treatment after film formation enables hydrogen, a hydroxyl group, or moisture to be released and removed from the oxide semiconductor film. In this manner, the field-effect mobility can be improved. Such an improvement in field-effect mobility is presumed to be achieved not only by removal of impurities by dehydration or dehydrogenation but also by a reduction in interatomic distance due to an increase in density. The oxide semiconductor can be crystallized by being highly purified by removal of impurities from the oxide semiconductor. In the case of using such a highly purified non-single-crystal oxide semiconductor, ideally, a field-effect mobility exceeding 100 cm²/Vsec is expected to be realized.

The oxide semiconductor containing In, Sn, and Zn as main components may be crystallized in the following manner: oxygen ions are implanted into the oxide semiconductor, hydrogen, a hydroxyl group, or moisture contained in the oxide semiconductor is released by heat treatment, and the oxide semiconductor is crystallized through the heat treatment or by another heat treatment performed later. By such crystallization treatment or recrystallization treatment, a non-single-crystal oxide semiconductor having favorable crystallinity can be obtained.

The intentional heating of the substrate during film formation and/or the heat treatment after the film formation contributes not only to improving field-effect mobility but also to making the transistor normally off. In a transistor in which an oxide semiconductor film that contains In, Sn, and Zn as main components and is formed without heating a substrate intentionally is used as a channel formation region, the threshold voltage tends to be shifted negatively. However, when the oxide semiconductor film formed while heating the substrate intentionally is used, the problem of the negative shift of the threshold voltage can be solved. That is, the threshold voltage is shifted so that the transistor becomes normally off; this tendency can be confirmed by comparison between FIGS. 20A and 20B.

Note that the threshold voltage can also be controlled by changing the ratio of In, Sn, and Zn; when the composition ratio of In, Sn, and Zn is 2:1:3, a normally-off transistor is expected to be formed. In addition, an oxide semiconductor film having high crystallinity can be obtained by setting the composition ratio of a target to In:Sn:Zn=2:1:3.

The temperature of the intentional heating of the substrate or the temperature of the heat treatment is greater than or equal to 150° C., preferably greater than or equal to 200° C., more preferably greater than or equal to 400° C. When film formation or heat treatment is performed at a high temperature, the transistor can be normally off.

By intentionally heating the substrate during film formation and/or by performing heat treatment after the film formation, the stability against a gate-bias stress can be increased. For example, when a gate bias is applied with an intensity of 2 MV/cm at 150° C. for one hour, drift of the threshold voltage can be less than ±1.5 V, preferably less than ±1.0 V.

A BT test was performed on the following two transistors: Sample 1 on which heat treatment was not performed after formation of an oxide semiconductor film, and Sample 2 on which heat treatment at 650° C. was performed after formation of an oxide semiconductor film.

First, V_(g)−I_(d) characteristics of the transistors were measured at a substrate temperature of 25° C. and V_(d) of 10 V. Note that V_(d) refers to drain voltage (a potential difference between a drain and a source). Then, the substrate temperature was set to 150° C. and V_(d) was set to 0.1 V. After that, −20 V of V_(g) was applied so that the intensity of an electric field applied to gate insulating layers was −2 MV/cm, and the condition was kept for one hour. Next, V_(g) was set to 0 V. Then, V_(g)−I_(d) characteristics of the transistors were measured at a substrate temperature of 25° C. and V_(d) of 10 V. This process is called a positive BT test.

In a similar manner, first, V_(g)−I_(d) characteristics of the transistors were measured at a substrate temperature of 25° C. and V_(d) of 10 V. Then, the substrate temperature was set to 150° C. and V_(d) was set to 0.1 V. After that, −20 V of V_(g) was applied so that the intensity of an electric field applied to the gate insulating layers was −2 MV/cm, and the condition was kept for one hour. Next, V_(g) was set to 0 V. Then, V_(g)−I_(d) characteristics of the transistors were measured at a substrate temperature of 25° C. and V_(d) of 10 V. This process is called a negative BT test.

FIGS. 21A and 21B show a result of the positive BT test of Sample 1 and a result of the negative BT test of Sample 1, respectively. FIGS. 22A and 22B show a result of the positive BT test of Sample 2 and a result of the negative BT test of Sample 2, respectively.

The amount of shift in the threshold voltage of Sample 1 due to the positive BT test and that due to the negative BT test were 1.80 V and −0.42 V, respectively. The amount of shift in the threshold voltage of Sample 2 due to the positive BT test and that due to the negative BT test were 0.79 V and 0.76 V, respectively. It is found that, in each of Sample 1 and Sample 2, the amount of shift in the threshold voltage between before and after the BT tests is small and the reliability thereof is high.

The heat treatment can be performed in an oxygen atmosphere; alternatively, the heat treatment may be performed first in an atmosphere of nitrogen or an inert gas or under reduced pressure, and then in an atmosphere containing oxygen. Oxygen is supplied to the oxide semiconductor after dehydration or dehydrogenation, whereby an effect of the heat treatment can be further increased. As a method for supplying oxygen after dehydration or dehydrogenation, a method in which oxygen ions are accelerated by an electric field and implanted into the oxide semiconductor film may be employed.

A defect due to oxygen deficiency is easily caused in the oxide semiconductor or at an interface between the oxide semiconductor and a film in contact with the oxide semiconductor; however, when excess oxygen is contained in the oxide semiconductor by the heat treatment, oxygen deficiency caused constantly can be compensated for with excess oxygen. The excess oxygen is oxygen existing mainly between lattices. When the concentration of excess oxygen is set to greater than or equal to 1×10¹⁶/cm³ and less than or equal to 2×10²⁰/cm³, excess oxygen can be contained in the oxide semiconductor without causing crystal distortion or the like.

When heat treatment is performed so that at least part of the oxide semiconductor includes crystal, a more stable oxide semiconductor film can be obtained. For example, when an oxide semiconductor film which is formed by sputtering using a target having a composition ratio of In:Sn:Zn=1:1:1 without heating a substrate intentionally is analyzed by X-ray diffraction (XRD), a halo pattern is observed. The formed oxide semiconductor film can be crystallized by being subjected to heat treatment. The temperature of the heat treatment can be set as appropriate; when the heat treatment is performed at 650° C., for example, a clear diffraction peak can be observed in an X-ray diffraction.

An XRD analysis of an In—Sn—Zn—O film was conducted. The XRD analysis was conducted using an X-ray diffractometer D8 ADVANCE manufactured by Bruker AXS, and measurement was performed by an out-of-plane method.

Sample A and Sample B were prepared and the XRD analysis was performed thereon. A method for manufacturing Sample A and Sample B will be described below.

An In—Sn—Zn—O film with a thickness of 100 nm was formed over a quartz substrate that had been subjected to dehydrogenation treatment.

The In—Sn—Zn—O film was formed with a sputtering apparatus with a power of 100 W (DC) in an oxygen atmosphere. An In—Sn—Zn—O target having an atomic ratio of In:Sn:Zn=1:1:1 was used as a target. Note that the substrate heating temperature in film formation was set to 200° C. A sample manufactured in this manner was used as Sample A.

Next, a sample manufactured by a method similar to that of Sample A was subjected to heat treatment at 650° C. As the heat treatment, heat treatment in a nitrogen atmosphere was first performed for one hour and heat treatment in an oxygen atmosphere was further performed for one hour without lowering the temperature. A sample manufactured in this manner was used as Sample B.

FIG. 25 shows XRD spectra of Sample A and Sample B. No peak derived from crystal was observed in Sample A, whereas peaks derived from crystal were observed when 2θ was around 35 deg. and at 37 deg. to 38 deg. in Sample B.

As described above, by intentionally heating the substrate during deposition of an oxide semiconductor containing In, Sn, and Zn as main components and/or by performing heat treatment after the deposition, characteristics of a transistor can be improved.

These substrate heating and heat treatment have an effect of preventing hydrogen and a hydroxyl group, which are unfavorable impurities for an oxide semiconductor, from being contained in the film or an effect of removing hydrogen and a hydroxyl group from the film. That is, an oxide semiconductor can be highly purified by removing hydrogen serving as a donor impurity from the oxide semiconductor, whereby a normally-off transistor can be obtained. The high purification of an oxide semiconductor enables the off-state current of the transistor to be less than or equal to 1 aA/μm. Here, the unit of the off-state current is used to indicate current per micrometer of a channel width.

FIG. 26 shows a relation between the off-state current of a transistor and the inverse of substrate temperature (absolute temperature) at measurement. Here, for simplicity, the abscissa represents a value (1000/T) obtained by multiplying an inverse of substrate temperature at measurement by 1000.

Specifically, as shown in FIG. 26, the off-state current can be less than or equal to 1 aA/μm (1×10⁻¹⁸ Aμm), less than or equal to 100 zA/μm (1×10⁻¹⁹ A/μm), and less than or equal to 1 zA/μm (1×10⁻²¹ A/μm) when the substrate temperature is 125° C. (398.15 K), 85° C. (358.15 K), and room temperature (27° C., 300.15 K), respectively. Preferably, the off-state current can be less than or equal to 0.1 aA/μm (1×10⁻¹⁹ A/μm), less than or equal to 10 zA/μm (1×10⁻²⁰ A/μm), and less than or equal to 0.1 zA/μm (1×10⁻²² A/μm) when the substrate temperatures is 125° C., 85° C., and room temperature, respectively.

Note that in order to prevent hydrogen and moisture from being contained in the oxide semiconductor film during formation thereof, it is preferable to increase the purity of a sputtering gas by sufficiently suppressing leakage from the outside of a film formation chamber and degasification through an inner wall of the film formation chamber. For example, a gas with a dew point of less than or equal to −70° C. is preferably used as the sputtering gas in order to prevent moisture from being contained in the film. In addition, it is preferable to use a target which is highly purified so as not to contain impurities such as hydrogen and moisture. Although it is possible to remove moisture from a film of an oxide semiconductor containing In, Sn, and Zn as main components by heat treatment, a film which does not contain moisture originally is preferably formed because moisture is released from the oxide semiconductor containing In, Sn, and Zn as main components at a higher temperature than from an oxide semiconductor containing In, Ga, and Zn as main components.

The relation between the substrate temperature and electric characteristics of a transistor formed using Sample B, on which heat treatment at 650° C. was performed after formation of the oxide semiconductor film, was evaluated.

The transistor used for the measurement has a channel length L of 3 μm, a channel width W of 10 μm, Lov of 0 μm, and dW of 0 μm. Note that V_(d) was set to 10 V. Note that the substrate temperature was −40° C., −25° C., 25° C., 75° C., 125° C., and 150° C. Here, in a transistor, the width of a portion where a gate electrode overlaps with one of a pair of electrodes is referred to as Lov, and the width of a portion of the pair of electrodes, which does not overlap with an oxide semiconductor film, is referred to as dW.

FIG. 23 shows the V_(g) dependence of I_(d) (a solid line) and field-effect mobility (a dotted line). FIG. 24A shows a relation between the substrate temperature and the threshold voltage, and FIG. 24B shows a relation between the substrate temperature and the field-effect mobility.

From FIG. 24A, it is found that the threshold voltage gets lower as the substrate temperature increases. Note that the threshold voltage is decreased from 1.09 V to −0.23 V in the range from −40° C. to 150° C.

From FIG. 24B, it is found that the field-effect mobility gets lower as the substrate temperature increases. Note that the field-effect mobility is decreased from 36 cm²/Vs to 32 cm²/Vs in the range from −40° C. to 150° C. Thus, it is found that variation in electric characteristics is small in the above temperature range.

In a transistor in which such an oxide semiconductor containing In, Sn, and Zn as main components is used as a channel formation region, a field-effect mobility of greater than or equal to 30 cm²/Vsec, preferably greater than or equal to 40 cm²/Vsec, more preferably greater than or equal to 60 cm²/Vsec can be obtained with the off-state current maintained at less than or equal to 1 aA/μm, which can achieve on-state current needed for an LSI. For example, in an FET where L/W is 33 nm/40 nm, an on-state current of greater than or equal to 12 μA can flow when the gate voltage is 2.7 V and the drain voltage is 1.0 V. In addition, sufficient electric characteristics can be ensured in a temperature range needed for an operation of a transistor. With such characteristics, an integrated circuit having a novel function can be realized without decreasing the operation speed even when a transistor including an oxide semiconductor is also provided in an integrated circuit formed using a Si semiconductor.

Example 1

In this example, an example of a transistor in which an In—Sn—Zn—O film is used as an oxide semiconductor film will be described with reference to FIGS. 27A and 27B.

FIGS. 27A and 27B are a top view and a cross-sectional view of a coplanar transistor having a top-gate top-contact structure. FIG. 27A is the top view of the transistor. FIG. 27B illustrates cross section A-B along dashed-dotted line A-B in FIG. 27A.

The transistor illustrated in FIG. 27B includes a substrate 1100; a base insulating film 1102 provided over the substrate 1100; a protective insulating film 1104 provided in the periphery of the base insulating film 1102; an oxide semiconductor film 1106 provided over the base insulating film 1102 and the protective insulating film 1104 and including a high-resistance region 1106 a and low-resistance regions 1106 b; a gate insulating layer 1108 provided over the oxide semiconductor film 1106; a gate electrode 1110 provided to overlap with the oxide semiconductor film 1106 with the gate insulating layer 1108 provided therebetween; a sidewall insulating film 1112 provided in contact with a side surface of the gate electrode 1110; a pair of electrodes 1114 provided in contact with at least the low-resistance regions 1106 b; an interlayer insulating film 1116 provided to cover at least the oxide semiconductor film 1106, the gate electrode 1110, and the pair of electrodes 1114; and a wiring 1118 provided to be connected to at least one of the pair of electrodes 1114 through an opening formed in the interlayer insulating film 1116.

Although not illustrated, a protective film may be provided to cover the interlayer insulating film 1116 and the wiring 1118. With the protective film, a minute amount of leakage current generated by surface conduction of the interlayer insulating film 1116 can be reduced and thus the off-state current of the transistor can be reduced.

Example 2

In this example, another example of a transistor in which an In—Sn—Zn—O film is used as an oxide semiconductor film and which is different from the above will be described.

FIGS. 28A and 28B are a top view and a cross-sectional view which illustrate a structure of a transistor manufactured in this example. FIG. 28A is the top view of the transistor. FIG. 28B is the cross-sectional view along dashed-dotted line A-B in FIG. 28A.

The transistor illustrated in FIG. 28B includes a substrate 600; a base insulating film 602 provided over the substrate 600; an oxide semiconductor film 606 provided over the base insulating film 602; a pair of electrodes 614 in contact with the oxide semiconductor film 606; a gate insulating layer 608 provided over the oxide semiconductor film 606 and the pair of electrodes 614; a gate electrode 610 provided to overlap with the oxide semiconductor film 606 with the gate insulating layer 608 provided therebetween; an interlayer insulating film 616 provided to cover the gate insulating layer 608 and the gate electrode 610; wirings 618 connected to the pair of electrodes 614 through openings formed in the interlayer insulating film 616; and a protective film 620 provided to cover the interlayer insulating film 616 and the wirings 618.

As the substrate 600, a glass substrate can be used. As the base insulating film 602, a silicon oxide film can be used. As the oxide semiconductor film 606, an In—Sn—Zn—O film can be used. As the pair of electrodes 614, a tungsten film can be used. As the gate insulating layer 608, a silicon oxide film can be used. The gate electrode 610 can have a layered structure of a tantalum nitride film and a tungsten film.

The interlayer insulating film 616 can have a layered structure of a silicon oxynitride film and a polyimide film. The wirings 618 can each have a layered structure in which a titanium film, an aluminum film, and a titanium film are formed in this order. As the protective film 620, a polyimide film can be used.

Note that in the transistor having the structure illustrated in FIG. 28A, the width of a portion where the gate electrode 610 overlaps with one of the pair of electrodes 614 is referred to as Lov. Similarly, the width of a portion of the pair of electrodes 614, which does not overlap with the oxide semiconductor film 606, is referred to as dW.

This application is based on Japanese Patent Application serial no. 2010-176963 filed with Japan Patent Office on Aug. 6, 2010 and Japanese Patent Application serial no. 2011-108049 filed with Japan Patent Office on May 13, 2011, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A semiconductor device comprising: a potential switching circuit; a first memory cell and a second memory cell adjacent to the first memory cell, each of the first memory cell and the second memory cell comprising: a first transistor, the first transistor being an n-channel transistor; a second transistor, one of a source electrode and a drain electrode of the second transistor being electrically connected to a gate electrode of the first transistor; and a capacitor, one electrode of the capacitor being electrically connected to the gate electrode of the first transistor, a source line electrically connected to one of the source electrode and the drain electrode of the first transistor of each of the first memory cell and the second memory cell, and the potential switching circuit; a first bit line electrically connected to the other of the source electrode and the drain electrode of the first transistor of the first memory cell, and the other of the source electrode and the drain electrode of the second transistor of the first memory cell; a second bit line electrically connected to the other of the source electrode and the drain electrode of the first transistor of the second memory cell, and the other of the source electrode and the drain electrode of the second transistor of the second memory cell; a first word line electrically connected to a gate electrode of the second transistor of each of the first memory cell and the second memory cell; and a second word line electrically connected to another electrode of the capacitor of each of the first memory cell and the second memory cell, wherein a channel formation region of the first transistor of each of the first memory cell and the second memory cell comprises a semiconductor material other than an oxide semiconductor, and wherein a channel formation region of the second transistor of each of the first memory cell and the second memory cell comprises an oxide semiconductor layer.
 2. A method for driving a semiconductor device, comprising the step of: supplying a power supply potential to a source line connected to one of a source electrode and a drain electrode of a first transistor, when a second transistor is turned on and a potential at which the first transistor is turned on is supplied to a node, wherein the node is formed by electrically connecting a gate electrode of the first transistor, one of a source electrode and a drain electrode of the second transistor, and one electrode of a capacitor, and wherein the node is configured to hold electric charge.
 3. The method for driving a semiconductor device according to claim 2, wherein a channel formation region of the first transistor comprises a semiconductor material different from a semiconductor material of a channel formation region of the second transistor.
 4. The method for driving a semiconductor device according to claim 2, wherein the channel formation region of the second transistor comprises an oxide semiconductor.
 5. The method for driving a semiconductor device according to claim 2, wherein the step of supplying the power supply potential to the source line is performed by controlling a potential of the source line with the use of a potential switching circuit.
 6. The method for driving a semiconductor device according to claim 2, wherein the source line is further connected to one of a source electrode and a drain electrode of a third transistor.
 7. The method for driving a semiconductor device according to claim 6, wherein each of the first transistor and the third transistor is an n-channel transistor.
 8. The method for driving a semiconductor device according to claim 6, wherein the first transistor and the second transistor are included in a first memory cell, and wherein the third transistor is included in a second memory cell adjacent to the first memory cell.
 9. The semiconductor device according to claim 1, wherein the potential switching circuit is configured to supply a power supply potential to the source line in a writing period.
 10. The semiconductor device according to claim 1, wherein the oxide semiconductor layer comprises In, Ga, Zn and oxygen.
 11. A semiconductor device comprising: a potential switching circuit; a first memory cell and a second memory cell adjacent to the first memory cell, each of the first memory cell and the second memory cell comprising: a first transistor, the first transistor being an n-channel transistor; a second transistor, one of a source electrode and a drain electrode of the second transistor being electrically connected to a gate electrode of the first transistor; and a capacitor, one electrode of the capacitor being electrically connected to the gate electrode of the first transistor, a source line electrically connected to one of the source electrode and the drain electrode of the first transistor of each of the first memory cell and the second memory cell, and the potential switching circuit; a first bit line electrically connected to the other of the source electrode and the drain electrode of the first transistor of the first memory cell, and the other of the source electrode and the drain electrode of the second transistor of the first memory cell; a second bit line electrically connected to the other of the source electrode and the drain electrode of the first transistor of the second memory cell, and the other of the source electrode and the drain electrode of the second transistor of the second memory cell; a first word line electrically connected to a gate electrode of the second transistor of each of the first memory cell and the second memory cell; and a second word line electrically connected to another electrode of the capacitor of each of the first memory cell and the second memory cell, wherein a channel formation region of the first transistor of each of the first memory cell and the second memory cell comprises crystalline silicon, and wherein a channel formation region of the second transistor of each of the first memory cell and the second memory cell comprises an oxide semiconductor layer.
 12. The semiconductor device according to claim 11, wherein the potential switching circuit is configured to supply a power supply potential to the source line in a writing period.
 13. The semiconductor device according to claim 11, wherein the oxide semiconductor layer comprises In, Ga, Zn and oxygen.
 14. The semiconductor device according to claim 11, wherein the channel formation region of the first transistor of each of the first memory cell and the second memory cell is formed within a substrate.
 15. A semiconductor device comprising: a potential switching circuit; a first memory cell and a second memory cell adjacent to the first memory cell, each of the first memory cell and the second memory cell comprising: a first transistor, the first transistor being an n-channel transistor; a second transistor, one of a source electrode and a drain electrode of the second transistor being electrically connected to a gate electrode of the first transistor; and a capacitor, one electrode of the capacitor being electrically connected to the gate electrode of the first transistor, a source line electrically connected to one of the source electrode and the drain electrode of the first transistor of each of the first memory cell and the second memory cell, and the potential switching circuit; a first bit line electrically connected to the other of the source electrode and the drain electrode of the first transistor of the first memory cell, and the other of the source electrode and the drain electrode of the second transistor of the first memory cell; a second bit line electrically connected to the other of the source electrode and the drain electrode of the first transistor of the second memory cell, and the other of the source electrode and the drain electrode of the second transistor of the second memory cell; a first word line electrically connected to a gate electrode of the second transistor of each of the first memory cell and the second memory cell; a second word line electrically connected to another electrode of the capacitor of each of the first memory cell and the second memory cell; and an insulating layer provided between the first transistor and the second transistor, and between the first transistor and the capacitor, wherein a channel formation region of the first transistor of each of the first memory cell and the second memory cell comprises crystalline silicon, and wherein a channel formation region of the second transistor of each of the first memory cell and the second memory cell comprises an oxide semiconductor layer.
 16. The semiconductor device according to claim 15, wherein the potential switching circuit is configured to supply a power supply potential to the source line in a writing period.
 17. The semiconductor device according to claim 15, wherein the oxide semiconductor layer comprises In, Ga, Zn and oxygen.
 18. The semiconductor device according to claim 15, wherein the channel formation region of the first transistor of each of the first memory cell and the second memory cell is formed within a substrate. 